The acceleration-time (a-t) graph for a particle moving along a straight line starting from rest is shown in figure. Which of the following graph is the best representation of variation of its velocity (v) with time (t)?

Given acceleration-time graph is shown in figure.Slope of tangent drawn to the a- t graph is instantaneous velocity of the particle .From the given diagram it is clear that ‘a’ is decreasing with time and becomes zero at some value of t .When a=0 , slope of tangent drawn to the v-t graph must be zero .These conditions are satisfied by graph (1) .