A pronoun must agree in number with the noun (or other pronoun) it refers to. Plural nouns take plural pronouns, and singular nouns take singular pronouns.
For example, this sentence has improper noun-pronoun agreement:
Citrus is a singular noun, so using plural pronouns to refer to it is incorrect. It would be correct to say:
Another problem with pronouns is unclear references. To know whether a pronoun agrees with its subject, you have to be clear about just what the pronoun refers to. For example, it’s not clear which noun the pronoun in this sentence refers to:
Because the subject of the first clause is plural, the pronoun he could refer to either Bobby or Tom or even to a third person. To improve clarity in this case, you use the name of the person who bought the candy bar rather than the ambiguous pronoun.
If a GMAT sentence-correction question contains a pronoun in the underlined portion, make sure the pronoun clearly refers to a particular noun in the sentence and that it matches that noun in number. Otherwise, you need to find an answer choice that clarifies the reference or corrects the number.
Much work performed by small business owners, like managing human relations, keeping track of accounts, and paying taxes, which are essential to its successful operation, have gone virtually unnoticed by their employees.
The underlined portion contains several agreement errors. To find them, isolate the main elements of this sentence. The subject is work. The main verb is have gone. The other verb (are) belongs to the dependent clause, so it can’t be the main verb.
So, the essential sentence states that work have gone unnoticed. This is not correct. You have to change the verb to the singular has to make it agree with the singular subject work. Eliminate any answer choices that don't change have to has, which leaves you with Choices (D) and (E).
Both Choice (D) and Choice (E) contain the verb are. So, the pronoun which must refer back to managing, keeping, and paying (which, together, are plural), so the verb that corresponds to which has to be plural, too. Also, both choices eliminate its before successful operation because it's unclear what its refers to.
The difference between the two choices is that Choice (E) changes their to its. Which noun the pronoun before employees refers to. Who or what has the employees? The only possibility is business owners, which is a plural noun. So the pronoun that refers to it must also be plural. Their is plural; its is singular. Therefore, Choice (D) is the best answer.