The correct answer is D. No error.
The sentence is grammatically correct. The subject “they” agrees with the verb “enjoyed” in number and person. The adverb
“thoroughly” modifies the verb “enjoyed”. The prepositional phrase “at the party” tells where the people enjoyed themselves.A. They
The subject “they” is a plural pronoun that agrees with the verb “enjoyed” in number.
B. enjoyed thoroughly
The verb “enjoyed” is a past tense verb that agrees with the subject “they” in tense. The adverb “thoroughly” modifies the verb “enjoyed” and tells how much
the people enjoyed themselves.C. at the party
The prepositional phrase “at the party” tells where the people enjoyed themselves.