Select the option that will improve the highlighted part of the given sentence. In case no improvement is needed, select 'No improvement required'.
My son has been sleeping for twelve hours at a stretch.

Correct option is D

The correct substitution of the highlighted words is (d) No improvement required.
Explanation: The present perfect continuous tense “has been sleeping” is correctly used here for an action that started in the past and is continuing in the present. Since the subject “My son” is singular, “has” is appropriate.
Grammatical rule used:
Present perfect continuous tense structure:
Subject + has/have been + verb + ing
· "Has" is used with singular subjects.
· "Have" is used with plural subjects or with “I/you”.
Example:
She has been reading for two hours.
Incorrect options:
· (a) “is been sleeping” – grammatically incorrect structure
· (b) “were sleeping” – past continuous, does not suit the current duration
· (c) “have been sleeping” – wrong subject-verb agreement with singular noun “son”