is the pronoun in parenthisis a subject or object pronoun (I) offered to buy supper for my best friend

The pronoun in parenthesis is the subject of the sentence.

Explanation:

The subject of the sentence is what the sentence is about - what performs the action represented by the verb or is being described.

The object is the receiver of the action represented by the verb.

I in the given sentence is the subject. We can ask: I did what? I offered to buy supper for my best friend. The object in this sentence would be supper.

in june, we will have been married for eighteen months. (i am assuming this is what you want? the sentence, as you have presented it, is grammatically incorrect--   you want it corrected, yes? )

i would think its c but i'm not sure