The answer is (C). Firstly note that in a weaken EXCEPT question, the correct answer either strengthens OR neither strengthens nor weakens the argument, i.e. it remains neutral.
It is not necessarily true that the correct answer to a weaken EXCEPT question must strengthen. With this in mind, let us analyze and break down the argument:
P1: Cave paintings describe
current diets of painters.
P2: Painters needed to eat sea animals populating waters north of Norway. This requires a long journey to and from the islands.
P3: However, no picture unambiguously depicts such creatures = some or many pictures depict such creatures(i.e. sea animals) ambiguously.
Conclusion: Therefore, the theory that cave paintings describe the diets of painters is false.
Assumption 1: Only food available are sea animals.
Assumption 2: The long journey to and from islands was so long that it would make it unfeasible for the hunters/painters to hunt sea animals.
Assumption 3: The paintings that were discovered is representative of all paintings found in the island, i.e. no painting was left undiscovered.
Now, on to the choices:
(A) Once on these islands, the cave painters hunted and ate land animals.
This definitely weakens one of our assumptions.
(B) Parts of the cave paintings on the islands did not survive the centuries.
This shows that the evidence provided might be limited or unrepresentative, and therefore weakens the
argument. Maybe they did paint sea animals, but those paintings have been lost.(D) Those who did the cave paintings that were discovered on the islands had unusually advanced techniques of preserving meats.
This weakens assumption 2, mentioned above.(E) The cave paintings on the islands were done by the original inhabitants of the islands who ate the meat of land animals.
This directly weakens the premise that the paintings describe current diets.Thus, by process of elimination, answer must be C. It is easier to prove why a choice weakens the argument rather than why a choice does not weaken an argument.
C) The cave paintings that were discovered on the islands
depicted many land animals.
If you go by assumption 1, mentioned above, this choice is definitely a weakener and therefore could bait you into thinking that this is not the correct answer. However, unlike choice A, choice C says "depicted" land animals, not hunted and ate. They may not prefer land animals to sea animals because maybe they were more dangerous to hunt. They could have figured out a way to hunt sea animals and preserved it (choice D) so as to not make frequent long trips. For these reasons, Choice (C) can also strengthen the conclusion, i.e. that they did not eat land animals and therefore that paintings are not based on diet.
Thus choice (C) can either be a weakener or a strengthener. It has an uncertain bearing on the conclusion and is therefore the correct answer.