Concept:The correct word must explain why the students could not understand the lecture.
Explanation:A lecture can only be described as unintelligible when its language or delivery cannot be understood.
The sentence clearly shows that the students did not understand what the lecturer said.
"Unintelligible" means impossible to understand or make sense of.
"Indefinite" means not clearly defined or not fixed in length, so it does not fit this situation.
"Illegible" refers to handwriting that cannot be read, not a spoken lecture.
"Unimpressive" means not interesting or not making a strong impression, which is not the reason given here.
Therefore, the lecture must have been unintelligible to the students.
Answer:C. unintelligible