Weaken

Thank you for your question @spk2018 E is incorrect because the conclusion is about whether language skills were needed to create the tools that Neanderthals made. The conclusion says nothing about the tools made by humans living at the same time as Neanderthals. E is not relevant. C is correct because if the tools made in the experiment were much simpler than those made by Neanderthals, then the tools made in the experiment would not be comparable to those made by Neanderthals. Since the tools would not be comparable comparable, we could not use the experiment to draw any conclusions about the Neandrethals' tools. The undergraduates did not need verbal instructions to make their simple tools, but if the Neanderthals' tools were more complicated then making them may have required verbal instruction.

Along these lines, why is D incorrect? I chose C, but had a hard time deciding between C and D.

Let's take a look at D, which tells us that the silent instructor was worse than the one in the speaking group. This means that all else being equal we would expect the silent group to have done worse. The fact that they did equally well indicates that the silence may actually have helped them. This actually strengthens the argument. If they were better silent, this suggests that it is in fact likely that Neanderthals were able to make their tools without language.