Select the most appropriate option to substitute the highlighted segment in the given sentence. In case no substitution is needed, select 'No substitution required'.
There are some people who think they are God’s gift to mankind.

Correct option is D

The correct substitution of the highlighted words is (d) No substitution required.
Explanation:
The sentence is grammatically and contextually correct. The relative clause "who think they are God's gift to mankind" properly modifies “people”. The verb “are” agrees with “they” in the present tense, suitable for a general truth or habitual belief.
Grammatical rule used:
When referring to people, use “who” (not “which” or “that”, although “that” is informally acceptable). Maintain tense consistency: habitual thoughts take present tense verbs like “think” and “are”.
Example:
People who think they’re always right often refuse to listen.
Why other options are incorrect:
(a) "whose thinking is..." – awkward and unnecessarily complex
(b) "which think..." – “which” is used for things, not people
(c) "that think they had been..." – incorrect past perfect usage for present opinion