Can velocity be zero while acceleration is not?

A tracking sensor records a cart moving along a straight rail. Its signed velocity is v(t) = 6 − 3t m/s, with t in seconds. The record is valid from t = 0 to t = 4 s.

At the instant the cart reverses direction, what is its acceleration?

Organize the givens & assumptions

Initial velocity: +6 m/s
Velocity changes by −3 m/s each second
Positive direction: along the initial motion

One-dimensional motion in an inertial laboratory frame; the stated velocity law holds throughout the interval.

A $0\ \mathrm{m\,s^{-2}}$
B $-3\ \mathrm{m\,s^{-2}}$
C $+3\ \mathrm{m\,s^{-2}}$
D $-6\ \mathrm{m\,s^{-2}}$

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