The main purpose of a cover letter is to:
a. get hired for a position.
b. be called in for an interview.
c. let your friends know how your job search is going.
d. obtain references from your network.

B: Be called for an interview

Explanation:

 The first step in a job application process is  to write and send a resumè, which contains the description of the applicant's job experience, education and accoplishments. After that, it is important to send a cover letter with more specific information, showing the qualifications and preference for this particular job, which may call attention of the hiring manager that could invite the person for an interview.