The correct and grammatically sound completion of the sentence is: "The applicant's poor writing skills was the principal reason he wasn't hired."
Therefore option a is correct.
In this sentence, the subject is "the applicant's poor writing skills," which is a singular noun phrase. Therefore, the verb that agrees with this singular subject should be "was." The correct use of the verb "was" ensures subject-verb agreement, as "skills" is a collective noun representing a singular concept in this context.
Additionally, the correct choice for the second blank is "principle." "Principle" refers to a fundamental truth or proposition, whereas "principal" typically relates to a person or thing of greatest importance. In the given sentence, the intended meaning is that the poor writing skills constituted the primary or fundamental reason for the applicant not being hired.
It's crucial to pay attention to subject-verb agreement and choose the right form of the verb based on the number of the subject. Similarly, selecting the appropriate homophone, such as "principle" over "principal," ensures precision in conveying the intended meaning.