They normally want the best answer. They are all possible, but "I have gained weight" is best because the others would more likely be followed by a different comment. The choice is too fine for a test question, I'd say.
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AlpheccaStars1, I am gaining weight. I need to stop eating so much. 2. I have gained weight. I have to lose some. 3, I have been gaining weight. I need to stop eating so much and go on a diet.4, I gained weight. I'd like to lose some.They are fine, but I could mix and match most of them.