C. in front of
The first sentence says that Helen is behind Mary which means that Mary is ahead of Helen. Thus, the answer must mean 'ahead of'.
Option A is incorrect. 'beside' means 'at the side of' and not 'ahead of'. Thus, it does not have the required meaning and is incorrect.
Option B is incorrect. 'between' means 'in the middle of' and not 'ahead of'. Thus, it does not have the required meaning and is incorrect.
Option C is correct. 'in front of' means 'ahead of'. Thus, it has the required meaning and is the correct answer.
Option D is incorrect. 'next' means 'nearest or adjacent' and not 'ahead of'. Thus, it does not have the required meaning and is incorrect.