The correct option is A Mona says that Indu is her best friend.
Direct speech is a sentence in which the actual words of the speaker are quoted.
Indirect speech is a sentence in where we report what someone said, without quoting his exact words.
If the reporting verb is in the Present Tense, the tense of the verb in the reported speech remains unchanged.
Here, option A is correct. As the reporting verb 'says' is in Present Tense, tense in reported verb remains unchanged. Pronoun 'my' changes to 'her' as Mona's is referring to her best friend.
In option B tense is changed and pronoun doesn't change. So, it is incorrect.
In option C tense is changed. So, it is incorrect.
In option D pronoun remains unchanged. So, it is incorrect.