Actually, they are all incorrect. It should be
in the next two weeks.
Now on to your actual question about tenses.
C) is incorrect. It uses future perfect tense, which is usually used in a context like this
By January, I will have lived here for 3 years.
You could say something like
When the test date comes, I will have studied hard for two weeks.
A) and B) are both correct, and they mean basically the same thing. Here's a link that explains the difference better than I could: https://english.stackexchange.com/questions/28234/will-be-doing-vs-will-do