fall-back

Fall-back, usually hyphenated as an adjective and often written as one word or two as a noun, describes something kept in reserve for use when a primary option fails or becomes unavailable.

A fall-back plan is what you resort to when Plan A doesn't work out.

Nuclear power became the fall-back when renewable energy couldn't meet demand.

The term suggests preparation for contingencies rather than preference—you hope you won't need it, but you're ready if you do.