The correct option is
C her
Option D: 'Her' is an object pronoun. It receives an action. In the given sentence, there are two objects, 'a bar of chocolate' is the direct object and the blank is an indirect object. Indirect objects answer the question 'to whom' or 'for whom?'.
The word 'her' is the most suitable word, as it answers the question 'for whom?'. Hence option D is correct.
Options A and C: 'For her' is also an indirect object, but we use 'for' when the indirect object is placed after the direct object. In the given sentence, it's placed before.
Similarly, 'to her' will also be used when the indirect object is placed after the direct.
Hence options A and C are incorrect.
Option B: 'Hers' is a possessive pronoun. It expresses possession of something. But here, the blank requires an object pronoun, because something 'bar of chocolate' is directed to that person 'her'. It does not express possession. Hence B is incorrect.