I can't see why A is not the correct answer - after all, Jill is relaying what Mary said, and she said 'in 10 days'. C is not correct. I assume that there is no more context than what has been provided? In which case, you can't know what day Mary spoke, therefore you can't know that her day of departure will be a Wednesday. Even if you did have additional information (that you have not provided here), which allows you to deduce that her day of departure is a Wednesday, C would not be correct - why? Because if the day that Mary made her statement is 10 days prior to the Wednesday of her departure, then there will be another Wednesday occurring before the departure Wednesday - so if you answer that she is going on Wednesday(without including the additional information that it is 10 days hence) then the listener will naturally assume that she will leave on the next Wednesday to occur, which will be one week prior to her actual departure.