The correct answer is: E. No error
The sentence “Every animal in the zoo is fed regularly and attended to very promptly” is grammatically correct. The subject “every animal in the zoo” is singular, so the verb “is fed” is also singular. The conjunction “and” joins the two independent clauses “is fed regularly” and “attended to very promptly.” The adverb “very promptly” modifies the verb “attended to.”
The sentence is clear and concise, and it conveys the intended meaning accurately.