Answer:
The pronoun in parenthesis is the subject of the sentence.
Step-by-step 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.