Keith has a 2018 tax liability of $2,250 before taking into account his American Opportunity tax credit. He paid $2,600 in qualifying expenses, was a full-time student, was not claimed as a dependent on his parents' return, and his American Opportunity tax credit was not subject to phase-out. What is the amount of his American Opportunity tax credit allowed?

a. $4,000

b. $2,250

c. $2,150

d. $0

e. $2,600