The correct answer is A. The union leader’s proposals could not be accepted by the manager.
The other options are incorrect because they do not accurately reflect the meaning of the sentence. Option B, “The union leader’s proposals were not accepted by the manager,” is in the past tense, but the sentence is in the present tense. Option C, “The union leader’s proposals will not be accepted by the manager,” is in the future tense, but the sentence does not specify when the manager will make a decision. Option D, “The union leader’s proposals would not be accepted by the manager,” is in the conditional tense, but the sentence does not express any doubt or uncertainty about the manager’s decision.
The sentence “The manager could not accept the union leader’s proposals” means that the manager is currently unable to accept the proposals, for whatever reason. It is possible that the manager may be able to accept the proposals in the future, but that is not certain.