Answer:
"The dress is a bit big on her, my mother has made it stay with pins."
Step-by-step explanation:
The 1st answer is not the answer because if you read it, it sounds like something someone would actually say.
The 2nd answer is not the answer because it makes sense when you read it. Semicolons are used to join two or more ideas (parts) in a sentence.
The 3rd answer is not the answer because turning them into 2 sentences is kind of irrelevant.