Question

which of these reform measures could voters use against a mayor they oppose?
a referendum
an impeachment
an initiative
a recall

Explanation:

A recall is a reform measure where voters can remove an elected official (like a mayor) from office before their term ends, which fits opposing a mayor. A referendum is about voting on laws/policies, impeachment is a legislative process (not direct voter - initiated for this context), and an initiative is about voters proposing laws, not removing officials.

Answer:

a recall