The correct option is
C Mine
When a pronoun in possessive case is placed before the noun being described it is called a possessive adjective. Example: This is my book. In this sentence, the word, 'my' is a possessive
adjective.
Sometimes, the pronoun in possessive case is used alone (and not before the noun). In such case, it is called a possessive pronoun. Example: This book is mine. Here, the word, 'mine' is a possessive pronoun.
In the given sentence, the noun being described is "friend". The pronoun (blank space) is away from the noun. Hence, we have to use the possessive pronoun.
Among the given options, the words, 'your', 'her' and 'him' are possessive adjectives, which are usually placed before the noun. Example: 'your friend', 'her friend' and 'his friend'. They are not used alone i.e away from the noun in any sentence. Hence options A, B, and C are incorrect.
The word, 'mine' is the only possessive pronoun (which can be used alone). Hence, option D is the correct answer.