mass of rope $=8 \times \large\frac{1}{2}$

$\qquad = 4 \;kg$

Total mass $=54 \;kg$

a-acceleration of system $=\large\frac{F}{m}$

$\qquad=\large\frac{108}{54}$$=2 m/s^2$

Force used for pulling rope $=4 \times 2=8\;N$

Therefore Force applied on mass $=108-8=100 \;N$

Hence b is the correct answer.