His lecture was … and so the students did not understand it.

Analyze the question

The sentence says the lecture was __ "and so the students did not understand it." We need a word that explains why the students failed to understand. That means the lecture could not be understood.

Explain the concept

This is a vocabulary-in-context question. The clue is the result clause: "did not understand it." The missing word must mean "not able to be understood."

Check each option

• A. indefinite: means vague or not clearly defined (e.g., an indefinite date). It is not the best fit for a spoken lecture, and it does not directly mean "impossible to understand."
• B. illegible: means not clear enough to be read, and it is used for handwriting or print. A lecture is spoken, so this does not fit.
• C. unintelligible: means impossible to understand. This matches the result clause exactly.
• D. unimpressive: means not impressive or not striking. A lecture can be unimpressive and still be understood, so it does not explain the students' confusion.

Answer

The best word is unintelligible, which is option C.

Tip

Look for the cause-and-effect link in the sentence. "And so" tells you the missing word is the cause of the result that follows. Also remember:

• Illegible is for what you read (handwriting).
• Unintelligible is for what you hear or comprehend (speech, explanations).