Hello:)!
I don’t really understand the uniform deceleration too :/

Deceleration is the opposite of acceleration.

The mathematical formula for deceleration is:

Uniform deceleration of an object can be defined as  decreasing the velocity of an object in equal amount in an equal interval of time.

The graph attached below shows a constant decrease in the velocity with a uniform time interval.

i think the answer is a, hope this : )