Geologists recently discovered marks that closely resemble worm tracks in a piece of sandstone. These marks were made more than half a billion years earlier than the earliest known traces of multicellular animal life. Therefore, the marks are probably the traces of geological processes rather than of worms.

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

(A) It is sometimes difficult to estimate the precise age of a piece of sandstone.
(B) Geological processes left a substantial variety of marks in sandstone more than half a billion years before the earliest known multicellular animal life existed.
(C) There were some early life forms other than worms that are known to have left marks that are hard to distinguish from those found in the piece of sandstone.
(D) At the place where the sandstone was found, the only geological processes that are likely to mark sandstone in ways that resemble worm tracks could not have occurred at the time the marks were made.
(E) Most scientists knowledgeable about early animal life believe that worms are likely to have been among the earliest forms of multicellular animal life on Earth, but evidence of their earliest existence is scarce because they are composed solely of soft tissue.

Source: LSAT
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Conclusion:
Marks are traces of GP(geological processes) rather than of Worms.

Pre-thinking:
Assumption: Traces of Worms/MultiCellularAnimals CANNOT pre-date the marking time on the sandstone.

Answer choice analysis of D and E:

(D) At the place where the sandstone was found, the only geological processes that are likely to mark sandstone in ways that resemble worm tracks could not have occurred at the time the marks were made.
Meaning: The GP(geological processes) that could have marked the sandstone DID NOT occur at the time when marks were ACTUALLY made.

Let's say:
Marks were marked on the sandstone in 1500 BC.
The GP(geological processes) that could have marked did NOT occur in 1500 BC.
Correct. This choice REMOVES the possibility that the marks are the trace of geological processes.

(E) Most scientists knowledgeable about early animal life believe that worms are likely to have been among the earliest forms of multicellular animal life on Earth, but evidence of their earliest existence is scarce because they are composed solely of soft tissue.

The trap-answer: Evidence of their earliest existence is scarce.
The scarcity of the evidence CAN sway in BOTH directions. - Let's say further evidences are found.
Strengthen:
Further evidence SHOWS that MultiCellularAnimals predate the marks.
Weaken:
Further evidence DOES NOT SHOW that MultiCellularAnimals predate the marks.

TakeAway:

A CORRECT weakener should NOT affect the conclusion after many more assumptions.
Any answer choice which CAN sway in BOTH the directions is NEVER a correct answer.

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Sneha333, A good thought!
However, even if one assumes that the place where sandstone was found was NOT the place of origin for choice D, i.e.,

Let's say:
The place of Origin - PofO
The place of Discovery - PofD

The sandstone could STILL be transferred from PofO to PofD because of natural events such as floods, landslides, etc. - A possibility

Even if we assume the above case( PofO != PofD ), it would NOT aid us in weakening the conclusion.

Conclusion: Marks are traces of GP(geological processes) rather than of Worms.
The geologists are concerned about the causes which could have led to those marking.

They are contemplating the probable origins of the marks. It does NOT matter whether sandstone's Discovery was the same place as its Origin.

To answer this:

Speaking for Option-D, I think the thought process behind this case( PofO != PofD ) was to look for an alternate cause such as:

1) the marks could have happened in the PofO rather than in PofD. &
2) the reason for those marks in PofO should NOT be GP(geological processes) - These things could have been true.

However, a lot of further assumptions are involved in reaching this aspect, which would NOT be correct unless backed by a coherent answer choice.

TakeAway:
A correct weakener:

1) Should be a piece of NEW information. - Not already stated/inferred.
2) Needs to raise a DOUBT on the conclusion. It need NOT necessarily destroy the conclusion.

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