People who have been drinking tend to their ability to tolerate alcohol.
a. understand
b. underestimate
c. overestimate

People who have been drinking end to overestimate their ability t tolerate alcohol  - c.

Indeed, this surge of confidence whilst drinking alchool has been often observed by many people and has also no doubt lead to many interesting encounters because of this overestimation. 

The physiological benefits of warming up and cooling down