Tony stark:
Pepper gave brith to you

Peter parker:
He saved you from a burning building and adopted you since brith parents died in the fire

Steve/captain America:
He was lonely so he went to the orphanage and adopted you.

Bucky:
You just walked up to him and you two had a staring contest and he soon said "your my child now" and you just nodded.

Bruce/Hulk:
Chemical accident. (Aka he made you from mixing the wrong chemicals by accident)

Loki:
He found you when he was out walking in the woods and you grabbed his cape as he walked pasted and he then decided to adopt you