Measurements of the motion of the planet Uranus seem to show

B it is..

it gives an alternate explanation than what is stated in the question stem.

ah, ok your explanations help a lot. i wasn't connecting the conclusion to the g-pull, the alternate explanation makes sense now. the whole time i was thinking that the answer had to do with the g-pull which i narrowed down to choice (b) and (d)

if b is correct then question must that 'comets exist beyond uranus' and not
' beyond the orbit of Pluto'
p n u sun

The argument tells us that Neptune and Pluto are beyond Uranus. Since they are not exerting the force on Uranus, the conclusion is that there is another planet beyond Pluto that we haven't yet discovered.
Option (B) tells that they there is a comet belt beyond Pluto with powerful pull. Hence, instead of a planet, the comets might be pulling Uranus away.

Answer B tells that it is not Neptune or Pluto, but some other comets that exert the pull on Uranus...
In weaking questions we can have a choice which gives another reason.
So (B)

Answer B provides the weakener we are looking for. Something else besides another planet is pulling the outer planets (Uranus, Neptune, and Pluto). Notice the stem only mentions the inner planets so all 3 of these outer planets are subject to this pull.

Hi Karishma,

But why isn't D good? D states that sun itself exerts lesser pull on uranus - so the cause is not the presence of a planet, its the absence of sun's pull itself...this very much undermines the argument...

eg. A jaguar is a very expensive car. Since John bought a jaguar, he mustv'e been helped financially by X.
Here the assumption is that everyone pays same for Jaguar. But if it so happens that the company charges very little from John, the argument stands weakened.
Though a statement such as: 'Z helps John too' also undermines the argument, I found the former better.

I really cant pinpoint the difference between B and D.

The issue at hand is "Measurements of the motion of the planet Uranus seem to show Uranus being tugged by a force pulling it away from the Sun and the inner planets." - the first line of the question. Uranus is being tugged by a force opposite to the direction of the Sun's pull. We are interested in the source of that pull. It doesn't matter how strong the Sun's pull is. If Sun's pull is weaker, the opposite force doesn't need to be very strong. If Sun's pull is stronger, the opposite force is even stronger. Bottom line, we need to find the source of the pull which is acting opposite to the Sun's pull. The argument says that the source of that pull cannot be the outer planets since their mass isn't enough. Hence there must be another planet beyond Pluto. But we undermine the argument by saying that there are comets with powerful pull beyond Pluto. So they could be the source rather than a planet yet to be discovered.

It is nowhere mentioned in the stimulus that some other than the sun object influences uranus movement .The stimulus says "seem to show Uranus being tugged by a force pulling it away from the Sun and the inner planets"
So what if no force that pulls uranus away actually exists , It is the sun that pulls all the all the planets toward itself . Uranus is the only planet for which the suns force is not strong enough

Argument:
Measurements of the motion of the planet Uranus seem to show Uranus being tugged by a force pulling it away from the Sun and the inner planets. Neptune and Pluto, the two known planets whose orbits are farther from the Sun than is the orbit of Uranus, do not have enough mass to exert the force that the measurements indicate. Therefore, in addition to the known planets, there must be at least one planet in our solar system that we have yet to discover.

Look at the colored portion above.. Measurements indicate that a force is acting to pull Uranus away from the Sun... (When no force acts on a body, it maintains straight line motion (or something like that, I dont remember my Physics well).. Sun acts on Uranus because of which Uranus revolves around the sun. How strong this force is doesn't matter because it is enough to make Uranus revolve around the Sun. If this force is less than that which Sun exerts on the inner planets, it doesn't bother us (as suggested in option D). The argument doesn't say that strength of the force that Sun exerts has changed or is decreasing.) Measurements are indicating another force acting against the Sun's force. So we are looking for the source of this force acting against the Sun's force.

Hi Karishma thanks for taking the time out to answer the question raised by a number of people repeatedly.
Lastly , can you tell us how should we consider the words "seem to " in bold in the quoted para above.Should we interpret "seem to " as apparently or actually.

'Measurements seem to show' means that 'measurements are indicating'... Whether there actually is a force pulling the planet or not is not established. It is a hypothesis based on the measurements.

e.g. Her behavior seems to show she is in trouble.
'indication received from her behavior'

If option D were: The force the Sun exerts on Uranus is decreasing every year or something, our conclusion might have been weakened. But D only compares the force exerted on Uranus with the force exerted on other planets which is irrelevant to us.

this is a simple B , took 1:03 mins , may be because i remember my physics very well. sometimes familiarity with the subject matter really helps, at other times some really awkward subject comes up and takes away too much time

OA added and bumping for further discussion

Measurements of the motion of the planet Uranus seem to show Uranus being tugged by a force pulling it away from the Sun and the inner planets. Neptune and Pluto, the two known planets whose orbits are farther from the Sun than is the orbit of Uranus, do not have enough mass to exert the force that the measurements indicate. Therefore, in addition to the known planets, there must be at least one planet in our solar system that we have yet to discover.

Which one of the following, if true, most seriously weakens the argument?

We have to find an option which mentions something is pulling Uranus but not planet

Option B conveys aptly

Posted from my mobile device

Measurements of the motion of the planet Uranus seem to show Uranus being tugged by a force pulling it away from the Sun and the inner planets. Neptune and Pluto, the two known planets whose orbits are farther from the Sun than is the orbit of Uranus, do not have enough mass to exert the force that the measurements indicate. Therefore, in addition to the known planets, there must be at least one planet in our solar system that we have yet to discover.

Which one of the following, if true, most seriously weakens the argument?

(A) Pluto was not discovered until 1930 - WRONG. Nothing done. Irrelevant.
(B) There is a belt of comets beyond the orbit of Pluto with powerful gravitational pull. - CORRECT. This may have lead to the pull opposite to Sun's.
(C) Neither Neptune nor Pluto is as massive as Uranus. - WRONG. Already said in passage.
(D) The force the Sun exerts on Uranus is weaker than the force it exerts on the inner planets. - WRONG. Scope shift and it's a trap.
(E) Uranus' orbit is closer to Neptune's orbit than it is to Pluto's. - WRONG. Again it does not impact the conclusion of the passage.

Answer B.