The two ways of relating what a person has said are 1)direct and 2)indirect (reported).

In the direct speech, the exact words of the original speaker are repeated.

In the indirect speech (reported speech), the exact meaning of what the speaker told is expressed but not necessarily in the speaker’s exact words.

Option C is correct as the direct speech is in the past continuous tense and the appropriate introductory verb (said) is used.

Options A,B, and D are wrong as the sentence is not transformed completely,'I' cannot be used, and 'have been' is not transformed respectively.

The correct answer is C)Mohan said that he had been studying very regularly since July.