A doctor may be able to diagnose(ক) a problem perfect(খ), but he may not be able to find a drug to which(গ) the patient will respond(ঘ).

Explanation:

Correct Answer is: perfect
Verb কে modify করে adverb এবং verb এর পরে adverb বসে। যেমন: He speaks fluently. `Diagnose` transitive verb (সকর্মক ক্রিয়া)। তাই Diagnose + object + adverb বসবে। Perfect এর adverb হলো perfectly.