Explanation:

- Original Sentence: Some people in the audience were asleep while the chief guest was speaking.

- This sentence correctly uses past continuous tense to describe an ongoing action (the chief guest speaking) that was happening at the same time as another action (people being asleep).

- Option 1: while a chief guest is speaking

- Uses present continuous tense, which is not suitable for a past event. Also, "a chief guest" is less specific than "the chief guest".

- Option 2: whenever the chief guest spoke

- Implies that every time the chief guest spoke, people fell asleep, which alters the meaning of the original sentence.

- Option 3: when the chief guest has spoken

- Uses present perfect tense, which is incorrect for a past event.

- Option 4: No substitution

- This option keeps the original sentence unchanged, which is correct for the context provided.