Step 1: Author identification.
The famous Nepali poet Laxmi Prasad Devkota wrote the poem "Koel."
Step 2: Option analysis.
(A) William Cowper: Incorrect.
(B) Alexander Pope: Incorrect.
(C) Laxmi Prasad Devkota: Correct.
(D) Puran Singh: Incorrect.
Step 3: Conclusion.
Correct answer is (C) Laxmi Prasad Devkota.