At last week's reception, most of the guests ____ tea rather than coffee.

Explication

A finished event at a stated past time (last week's reception) calls for the past simple, and for this verb that form is drank. Drunk is the past participle and works only after have or be, while drunken is no longer used as a verb form: it survives only as an adjective placed before a noun, as in a drunken argument.