(a.) Belonging or relating to volition.

additional agent alternate alternate choice analogy another attainable backup change changeling choice comparison contingency copy counterfeit deputy different discretion discretional disjunctive double druthers dummy election elective equal equivalent ersatz escape clause escape hatch exchange exchangeable fake fill-in ghost ghostwriter imitation interchangeable locum tenens loophole makeshift metaphor metonymy mock next best thing option optional optionality personnel phony pinch pinch hitter pleasure possibility possible choice preference pretext provisional proxy relief replacement replacing representative reserve reserves ringer saving clause second string secondary selection sign spare spares stand-in stopgap sub substituent substitute substitution succedaneum superseder supplanter surrogate symbol synecdoche temporary tentative third string token understudy utility utility player variant vicar vicarious vice-president vice-regent volitional voluntary way of escape way out will and pleasure