Select the most appropriate option that can substitute the underlined segment in the given sentence. If there is no need to substitute it, select ‘No substitution required’.
She was thinking at the fun they had.

The correct answer is about.

Explanation:
In English grammar, certain verbs pair with specific prepositions to express correct meanings. The verb "think", when referring to recalling memories, reflecting on an experience, or considering something in one's mind, is followed by the preposition "about" (or sometimes "of").

In the given sentence, "She was thinking at the fun they had," the preposition "at" is incorrect because "think at" is not a standard English idiom for reflecting on past experiences.

Replacing "at" with "about" forms the grammatically correct sentence:
"She was thinking about the fun they had."