This argument proceeds by saying that:

Premise 1: The largest volcano on mars rises 26 kilometres...

Premise 2: the Earth’s crust, although of essentially the same composition as that of Mars, is too thin to support even a small fraction of that mass

Conclusion: No volcano of such size could exist on Earth

Okay, so if Earth can't handle a volcano as big as the largest volcano on Mars, and practically the only reason it can't is because Earth's crust is too thin (remember--it's not the composition because that is the same between the two, and it's not gravity, because the question says as much), then to handle the largest volcano, Mars needs to have a crust (at least at that point) that is thicker than the crust of the earth. Otherwise Mars, too, wouldn't be able to handle the volcano.

(d) says this, and is the right answer. No other answer can be shown to be true on the basis of the question alone.
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argument
=if gravity in mars was like earth
=earth would still not be able to support
=volcanoes like those in mars
=bc e's crust is too thin

infer
ans (D) safe to assume that since such v's in mars would sink under e's crust, then m's crust must be at least thicker!

The largest volcano on Mars rises 26 Kilometers above the surrounding plain and covers an area roughly the size of Romania. Even if the Earth’s gravity were as low as the gravity of Mars, no volcano of such size could exist on Earth, for the Earth’s crust, although of essentially the same composition as that of the Mars, is too thin to support even a small fraction of that mass and would buckle under it, causing the mountain to sink.

IMO D because

(D) The crust of Mars, at least at certain points on the planet, is thicker than the crust of the Earth.

this is supported in the last line that
Earth’s crust is too thin to support even a small fraction of that mass which means that Mars crust is thicker to support( at least ) at that part to support a mountain of that size
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The passage tells us that earth cannot support such a large mountain because its crust is too thin.
But in Mars, this massive mountain easily exists.

So, what is that deciding factor that allows Mars to support such a massive volcano?

The only description of the earth's surface (as provided in the passage) is the thickness of its surface. So, this is the variable (the thickness of the surface) that should help us explain why Mars is able to support this massive volcano.

Only (D) talks about the surface thickness of Mars (specifically, D mentions that at least some of Mars' surface, if not all, is certainly thicker than the earth's surface). Hence, (D) is the right answer.

Can anyone explain what's wrong with option E

(E) At least some of Earth’s volcanoes would be larger than they actually are if the Earth’s crust were thicker than it is.

We don't know if this is the ONLY condition that the larger volcanoes could happen, maybe the mass would still sink the mountain even with such thickness (is thicker but not thick enough)

E) At least some of Earth’s volcanoes would be larger than they actually are if the Earth’s crust were thicker than it is.


There are 2 conditions for volcanoes to be larger on Earth:
1. Earth's gravity would have to be as low as Mars
2. Earth's crust has to be as thick as or thicker than Mars
Condition 1 is not satisfied and only half of condition 2 is satisfied without a comparison. The other half of point 2 is, thicker is insufficient and doesn't have an end comparison. How thick is the question.

E does seem the most attractive but has a trap. D is the most certain option.

(A) The surface of Mars is less subject to forces of erosion than is the surface of the Earth. -- Not borne out by the passage. Eliminate. The passage talks about "gravity" and "forces" of erosion could include elements other than "gravity" alone.

(B) The highest volcanoes on Mars occur where its crust is thickest. -- Not borne out by the passage. The passage only says that "thin" crust is the reason why Earth does not have that big volcanoes. This is too extreme an inference.

(C) On average, volcanoes on Mars are higher than those on Earth. -- Again, we do not know and this is an extreme inference. The passage only talks about a specific volcano.

(D) The crust of Mars, at least at certain points on the planet, is thicker than the crust of the Earth. -- This can be reasonably inferred from the passage. Since "thin" crust of earth is the reason for not supporting such volcanoes (assuming similar levels of gravity), then it is obvious that atleast at the spot of that 26km tall volcano, the crust would be "thicker" than the crust of earth.

(E) At least some of Earth’s volcanoes would be larger than they actually are if the Earth’s crust were thicker than it is. --- Contender. Honestly, while this may be true but we can't say for sure. We know what is the reason for volcanoes not being "very tall" on earth, but we can't reverse that logic and say if that is the "only" reason. The passage provides a reasoning relative to Mars.