Select the most appropriate option that can substitute the highlighted words in the given sentence.
Kiran’s teacher was doubtful on her success.

Correct option is D

The correct substitution of the highlighted words is (d) doubtful of.
Explanation: The correct preposition to use after “doubtful” when expressing uncertainty about something is “of”. Hence, the correct phrase is “doubtful of her success.”
Grammatical rule used: Adjective + preposition collocation – Certain adjectives are followed by specific prepositions, and “doubtful” is always followed by “of” when referring to doubts about a noun.
Example: She was doubtful of his ability to complete the task.
�� Booster:
Always use “doubtful of + noun/pronoun” to express uncertainty:
✅ “He was doubtful of the results.”
❌ “He was doubtful on/in/off the results.”
Meanings of incorrect options:
· (a) Doubtful beyond – Grammatically incorrect and awkward.
· (b) Doubtful in – Wrong prepositional collocation.
· (c) Doubtful off – Incorrect and ungrammatical.