The Right Answer

We can solve this with Math Strategy, using the comma directly following the blank. On the right-hand side, we can see that opposition to the ban has weakened, and the left-hand side should also convey this. The blank describes how the ban has changed, so if the opposition has weakened, we can fill the blank with something like "likely".

The best match for this blank is imminent, which suggests that the ban is "likely to happen soon".

The Wrong Answers

imprudent - This choice has no evidence. There's nothing indicating that the ban would not be a smart choice.
fortuitous - This choice has no evidence. There's nothing indicating that the ban would be coming at a fortunate time.
premature - This choice contradicts the logic. If the ban happened after the ozone hole was discovered, then it definitely didn't happen too early.
unlikely - This choice contradicts the logic. If the opposition had weakened, the ban would be more likely.