The correct answer is D. of.
Anya was very conscious of her weakness. This means that she was aware of her weakness and was constantly thinking about it. She may have been worried about her weakness, or she may have been trying to improve it.
The other options are not as accurate. Option A, by, is not a good choice because it suggests that Anya was controlled by her weakness. Option B, in, is not a good choice because it suggests that Anya was trapped in her weakness. Option C, for, is not a good choice because it suggests that Anya was grateful for her weakness.