The correct option is D I
Here, 'I' as first person pronoun is correct because it implies the one who is speaking or writing. Here, a friend and the person who is writing, they both went for a walk.
'Me' is incorrect because it is the form of I that is used when the speaker or writer is the object of a verb or preposition, or after the verb be.
'My' is incorrect because it implies of or belonging to the speaker or writer.
'Mine' is incorrect because it implies of or belonging to the person writing or speaking.