This question is a very close rewriting of OG-V2 #52. Please always post the question source so other students can gage the quality of the source. A quick google search reveals that this is a Veritas prep question - this is one of dozens of examples of Veritas questions that are plagiarizations of Official Guide questions.

But, to answer the question...

Whenever you're asked to find the ASSUMPTION, you must determine the conclusion of the argument, the reason given to support that conclusion, and what information is missing.

Conclusion: The interviews are the most important part of the process.
Reason: Without them, [the company] might occasionally hire people with personalities that are poorly suited to both the company and the specific position.
Missing information: The interviews must accurately reveal who has personalities that are poorly suited to the job.

When you're looking through the answers choices, look for the thing that is NECESSARY to the validity of the conclusion. Ask yourself - does this have to be true?

(a)The only purpose of a job interview is to assess the personality of an applicant.
This does not have to be the ONLY purpose of the interview in order for the interview to perform the function of determining personality fit. Answer choices with EXTREME LANGUAGE are usually wrong on assumption questions.

(b) An applicant is not able to hide personality traits that may be detrimental to the given position or company.
Does this have to be true? Yes! If they were able to hide such personality traits, then the interview would not be able to reveal candidates who were poorly suited to the job. This has to be true for the conclusion to hold.

(c) All interviewers at Lightwave Industries are capable of determining whether a candidate's personality is a good fit for the company and position.
This one is tricky, and I think it makes this a poorly-written (as well as plagiarized) question. You would think that it would be necessary that a) candidates can't hide their personality, and b) that interviewers can correctly assess personality. However, it does not have to be true that ALL interviewers can do this - only that in general, this can be done. However, I think this is far too close to a right answer, making this a bad question.

(d) Most companies agree that a good personality fit is the most important factor in the job application process.
It doesn't matter what most companies think. We only need to determine whether the conclusion holds for this company.

(e) Lightwave Industries is not a business for which having a good personality match is a particularly important hiring trait.
This would actually negate the validity of the argument - the interview would no longer be important for the reason stated. We need to support the conclusion, not undermine it.

The correct answer should be B. However, this is a bad question, and I recommend that you refrain from using this source.