Can a body decelerate while moving with constant speed? if so mention an example.
Deceleration is negative acceleration. Circular motion is the typical example where speed can be constant without acceleration (or negative deceleration) being zero.
In physics the term "deceleration" isn't really used. If an object that is moving in a straight line is speeding up, then it is experiencing a positive acceleration. And if it is slowing down, then it is still accelerating but now with a negative value.
Newton's equation F=ma (Force = mass x acceleration) must always be true. And we know that it takes a force to change the speed or direction of any object. So, if an object is speeding up, slowing down, or changing direction, and since the mass is a constant value, then to keep the equation balanced the value of acceleration simply can not be 0.
So, if you were to move at a constant speed "in a straight line" then you would not be accelerating. But... One quick example of moving at a constant speed where you would be experiencing acceleration is to ride a Merry-Go-Round.