Concept:The given sentence is an example of a third conditional, which refers to an unreal past situation. When converting such direct speech to indirect speech, the past perfect and "would have" structure must stay the same to keep the original meaning.
Explanation:In direct speech, the sentence uses “if you had prepared” (past perfect) and “you would have solved” (would have + past participle). The reporting verb “said” is past tense, so the tense in the reported clause normally shifts back. However, past perfect cannot shift further back, so it remains unchanged. Similarly, “would have solved” does not change.
Option A changes to “if they prepared” and “would solve”, which turns it into a second conditional (hypothetical present). Option C uses “would solve” instead of “would have solved”, altering the past result. Option D changes the condition to present tense and incorrectly mixes “would have solved”. Only Option B keeps both the past perfect condition and the past result intact.
Answer:Option B: “The professor said that if they had prepared more thoroughly they would have solved the problem easily.”