the correct choice is
b. negative acceleration
acceleration is given as rate of change of velocity and is given by the formula
a = (v - v₀)/t
where a = acceleration , v = final velocity, v₀ = initial velocity , t = time
as the car slows down , the final velocity is smaller than the initial velocity.
v < v₀
so (v - v₀) < 0
so acceleration = a = (v - v₀)/t
since (v - v₀) < 0
hence a < 0
so the acceleration is negative
