What can a cover letter explain that a résumé cannot?
a.
gaps in employment history
b.
reentry into the job market
c.
changes in career focus
d.
all of the above

I would have to say a all of the above, because a cover letter is just explaining why you would like to work there, it doesn't explain where your work history. b and c both are questions that you would explain in a cover letter.

Answer the above statement is (true);