The cell’s nucleus is like (A)/ the conductor of a huge (B)/ opera who tell all the (C)/ performers what to do (D)/.

The correct answer is Option C.

Let us read the full sentence carefully by joining all its parts:

"The cell's nucleus is like the conductor of a huge opera who tell all the performers what to do."

The question asks us to identify the part of the sentence that contains a grammatical error. Let us examine each part one by one.

Part A: "The cell's nucleus is like" — This is grammatically correct. The possessive "cell's" is properly formed, and "is like" is the correct verb phrase for a comparison.

Part B: "the conductor of a huge" — This is grammatically correct. It is a noun phrase with a prepositional modifier, and there is no error here.

Part C: "opera who tell all the" — This is where the error lies. The relative pronoun "who" here refers back to "the conductor", which is a singular noun. According to the rule of Subject-Verb Agreement, a singular subject must take a singular verb. Therefore, the verb "tell" (a plural/base form) is incorrect. It must be replaced with "tells" (the singular third-person form).

Part D: "performers what to do" — This is grammatically correct as the object clause following the verb.

The Error Explained (Subject-Verb Agreement):

The subject of the relative clause introduced by "who" is "the conductor" (singular). The verb in that clause must therefore agree in number with this singular subject.

❌ Incorrect: "...the conductor of a huge opera who tell all the performers..."

✅ Correct: "...the conductor of a huge opera who tells all the performers..."

A common trap here is that the phrase "of a huge opera" appears between the subject (conductor) and the verb (tell/tells), which can mislead the reader into thinking the subject is "opera" (which is singular too, but the verb would still need to be singular). The core rule remains: identify the true subject of the relative clause, which is "conductor" — and use the singular verb "tells" accordingly.

Therefore, the grammatical error is in Part C.