If a particle starts its motion from x=-4m, then which one represents the a-t graph?
The first figure is the question figure, the second figure contains the answer, and the third one contains the other options.
Options
Why is this the right answer 
Why are these the wrong answers?
HINT: In your figure--the one that has the velocity graph that is the question, what is the slope of the graph, first for $t$ in the interval $(0,2)$, then for $t$ in the interval $(2,4)$.
HINT 2: The slope of a line segment from say $[0,2]$ is always a constant i.e., it is the same at $1/2$ as it is at $3/2$.