Your "balloon being inflated" example doesn't quite fit. The question is not asking about the surface tension as the membrane distorted or after it distorted. Instead, the question is asking about the conditions that allowed the membrane to distort in the first place. As the solution explains, a lower surface tension would allow for stretching of the membrane.
I am not sure, I think what you said is a reasonable explanation, but the wording does not fit. If your interpretation is correct, I think the answer choice should say, "the membrane's surface tension is low" and the solution should say, "this means that the membrane's surface tension must be low"
Using the word "decreased" seems to imply that there was a change during the experiment.