Select the most appropriate option that can substitute the highlighted word in the given sentence. If there is no need to substitute it, select ‘No substitution required’.
The talk was not understandable.

Correct option is A

The correct answer is (a) no substitution required.
 In this sentence, the word "understandable" is used to describe the quality of the talk, meaning whether or not it could be understood easily.
 The sentence itself is grammatically correct and conveys the intended meaning.
 There is no need to replace the word "understandable" with any other word because it effectively communicates that the talk was not easy to understand.
 So, option (a) "No substitution required" is chosen because the original word "understandable" is suitable in the context of the sentence, and there is no need to replace it with any other word.