Figures (a), (b), (c) and (d) show variation of force with time. The impulse is highest in figure.

As we know, impulse is the area under the curve of the force vs. time graph. So, analysing each figure area under curve.

$\left(a\right) A_1=\frac{1}{2}\times 1.0\times 0.5=\frac{1}{4} \mathrm{N}‐\mathrm{s}$

$\left(b\right) A_2=0.5\times 2=1 \mathrm{N}‐\mathrm{s}$

$\left(c\right) A_3=\frac{1}{2}\times 1\times 0.75=\frac{3}{8} \mathrm{N}‐\mathrm{s}$

$\left(d\right) A_4=\frac{1}{2}\times 2\times 0.5=\frac{1}{2} \mathrm{N}‐\mathrm{s}$

From above calculations, area under curve in figure (b) is maximum. Thus, fig (b) impulse is highest.