Figures (a), (b), (c) and (d) show variation of force with time.

The impulse is highest in figure.

To compare impulses, remember one key idea:

Impulse = area under the F-t graph

Now evaluate each figure based on its shape and dimensions:

For graph a

This is a triangle with $$base\ 1s\ and\ hieght\ 0.5N$$.
Area (impulse) =

$$\frac{1}{2}\times\ 1\times\ 0.5=0.25N$$

For graph B

This is a rectangle with constant force $$0.5N\ \ for\ a\ time\ of\ 2s$$.
Area (impulse) =

$$0.5\times\ 2\ =1N$$

For graph C

Triangle with base 1 s and height 0.75 N
Area =

$$\frac{1}{2}\times\ 1\times\ 0.75=0.375N$$

For graph D

Triangle with base 2 s and height 0.5 
Area =$$\frac{1}{2}\times\ 2\times\ 0.5=0.5N$$

Therefore from this we can conclude that fig(b) has the highest impulse