The manager could not accept the union leader's proposals.

The union leader's proposals could not be accepted by the manager

The union leader's proposals were not accepted by the manager

The union leader's proposals will not be accepted by the manager

The union leader's proposals would not be accepted by the manager

All of the above

Explanation

The given sentence contains one of Model verb (Model Verb = will, shall, can, may, might, could, might, must, would). It is in active voice and a negative sentence.

Rule :
Subject + Model verb +not + be + V3 + Optional Objects