Option a  can be eliminated directly as the number of days have to be  $>$ 9 days.

Answer is either b,c and d

Check for answer b.

If slower one takes 16 days, then faster takes  $\frac{1}{4}-\frac{1}{16}=\frac{3}{16}\Rightarrow \frac{16}{3}$  days

Slower takes  $\frac{16}{2}=8$ days and faster takes $\frac{\left(\frac{16}{2}\right)}{3}=\frac{8}{3}$ days. Totally they will not take 9 days

Reverse Gear

Check for answer c

If the slower crusher takes 15 days, then the faster one takes $\frac{1}{4}-\frac{1}{15}=\frac{11}{60}\Rightarrow \frac{60}{11}$ days.

Slower does his half work in $\frac{15}{2}=7.5$ days and the faster does his work in $\frac{60}{22}$. together it does not add up to 9 days. Hence, answer is option (d)