Answer
Velocity will be
zero at $t=01,2,3,4 seconds$
Most positive at t=0.5 & 2.5 seconds
Most negative at t = 1.5 &3.5 seconds
Work Step by Step
Velocity of block if given by the tangent of the y-t graph. Where the slope is zero , means velocity is zero. Where it is positive or negative means velocity is positive or negative respectively.
Slope at $t=0, 1s,2s,3s,4 s$ is zero means velocity is zero at these points.
Slope at $t =0.5s,2.5s$ is most positive means velocity at least points will be most positive.
Slope at$t=1.5s,3.5s$ is most negative means velocity at these points is most negative.
Graph between v and t is as
