No, it is not possible for an object to have a constant speed and decreasing acceleration simultaneously.
Acceleration is defined as the rate of change of velocity with respect to time. If an object's acceleration is decreasing, it means that the rate of change of its velocity is decreasing over time. This implies that the object is either slowing down (decelerating) or changing direction.
If an object has a constant speed, it means that its velocity is not changing over time. In other words, the object is maintaining a consistent magnitude and direction of motion. In this case, the object's acceleration must be zero because there is no change in velocity.
So, an object cannot have both a constant speed and a decreasing acceleration at the same time. The two concepts are contradictory.