Answer: b. three or four
Step-by-step explanation:
Cover letters are meant to introduce the applicant for the role in a manner that would inspire the recruiter to want to take their applicant further in the recruitment process.
As such, the applicant should include some selling points to encourage this. One of said selling points is the possession of the skills that the role in question requires. A good number to possess/include in the letter is three to four of said skills because anything less might be too little and anything more might inspire distrust.