Extract:
Which word is used incorrectly in the following sentence?
The boys leapt up happy to assist their favorite teacher.
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A
leapt
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B
happy
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C
assist
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D
their
The word "happy" is used incorrectly; the adverb "happily" is required to modify the verb "leapt."
A) leapt
"Leapt" is correctly used as past tense verb (variant of "leaped"). No error affects this verb form.
B) happy
"Happy" is an adjective—cannot modify verbs. Adjectives modify nouns; adverbs modify verbs. Correct form is adverb "happily" ("The boys leapt up happily") to describe how they leapt.
C) assist
"Assist" is correctly used as base verb form after infinitive marker "to." No error affects this verb.
D) their
"Their" is correctly used as possessive pronoun modifying "favorite teacher." No error affects this pronoun.
Conclusion
Adjectives cannot modify verbs—only adverbs can. Since "happy" attempts to modify the action verb "leapt," the adverbial form "happily" is grammatically required to describe the manner of leaping.