The correct option is
C As
The correct answer is Option C.
AS is used to indicate something that happens during the time when something else is taking place.
In the given sentence, when she grows older, her confidence also grows together. Both the things are happening together.
Therefore AS is the correct answer.
The remaining options are incorrect because:
THAT and WHICH are used to describe something.
LIKE WHICH is grammatically incorrect.
WHO is used to describe a person.