To summarize the argument:
These people highly enjoy a drink that they drink in small amounts at dawn.
Anthropologists believe they do this because it's high caffeine would mess with there physical daily activities.
How do we disprove the anthropologists? To find this we only need to find another reason these people would drink low doses in the morning.
(A) is certainly not the answer. Why would they drink less of something that they need in their diet?
(B) this is irrelevant. We wont to prove why they don't drink much of it in the morning, not night.
(C) this is correct. This gives us a good reason why they only drink a little bit of the tea. Its not because they are worried about the caffeine. They don't want to have to do their daily activities high.
(D) again, this is irrelevant. Children may drink it in a weak form, but that doesn't give us a reason why they only drink a small amount in the morning.
(E) again, irrelevant. What does it matter that they drink it a lot when they celebrate. This doesn't provide us with any information as to why they would only drink a small amount in the morning.
I hope this helps.
Give me a kudos if it helped

Thanks,
Jared