Correct Answer: R

Solution : P can do $\frac{1}{4}$th of work in 10 days.
⇒ To complete the work, P will take 10 × 4 = 40 days
Q can do $\frac{2}{5}$th of work in 40 days.
⇒ To complete the work, Q will take 40 × $\frac{5}{2}$ = 100 days
R can do $\frac{1}{3}$rd of work in 13 days.
⇒ To complete the work, R will take 13 × 3 = 39 days
So, R will complete the work first.
Hence, the correct answer is R.