When an object is having variable velocity then we can use the definition of acceleration to find it that rate of change in velocity is known as acceleration.
We can define it as
[tex]a = \frac{v_f - v_i}{\Delta t}[/tex]
here
[tex]v_f[/tex] = final velocity
[tex]v_i[/tex] = initial velocity
[tex]\Delta t[/tex] = time interval
now it is given here that final velocity is less than initial velocity after some time which means
[tex]v_f < v_i[/tex]
so here we can say acceleration will have negative value which shows it is opposite to the direction of velocity.
So this is the case of decreasing velocity which is known as deceleration.
So correct answer is
deceleration
