Select the most appropriate option to fill in the blank.
________ I cannot understand is how you could walk so far.

Correct option is D

The correct option to fill in the blank is "What."
Explanation:
· The sentence structure suggests a noun clause where "What" acts as the subject of the sentence.
· Correct sentence: What I cannot understand is how you could walk so far.
· "What" introduces the thing that is not understood.
Incorrect Options:
· (a) When – Used for time-related questions (incorrect here).
· (b) How – Introduces manner, not the subject of the sentence.
· (c) Which – Used for selecting among specific choices.
Example:
· What worries me the most is the weather.