Question

what is the only effective way to reduce your blood alcohol content (bac)
exercising.
allow your body time to get rid of alcohol.
taking a cold shower.

Explanation:

The liver metabolizes alcohol at a relatively fixed rate. Exercise and cold - showers do not speed up this metabolic process. Only time allows the body to fully metabolize and eliminate alcohol, thus reducing BAC.

Answer:

Allow your body time to get rid of alcohol.