Explanation

the OA is D. Imminent means: likely to occur at any moment, soon. The tone of the sentence suggests that the CFC are dangerous and they are going to be depleted any time rapidly.

Considering also they the strong companies' opposition to produce such chemicals had weakened, the ban is imminent.

regards
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As the companies' opposition has weakened, the ban is most likely to happen. The answer is imminent because it means "about to happen".

With the 1985 discovery of an ozone hole over Antarctica, an international ban on the production of chlorofluorocarbons - implicated in causing the ozone hole - began to appear (i) __________ ,especially since chemical companies' opposition to such ban had weakened.

There are two facts to be noted here:

1. The 1985 discovery of an ozone hole over Antarctica
2. Opposition of chemical companies to such a ban had weakened.

Therefore, since chlorofluorocarbons have been implicated in causing the ozone hole, an international ban on the production of chlorofluorocarbons is very likely or imminent.
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