Elective

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late Middle English: from Old French electif, -ive, from late Latin electivus, from elect- ‘picked out’, from the verb eligere (see elect).

wiktionary

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elect +‎  -ive

etymonline

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elective (adj.)

early 15c., "voluntary," from Late Latin electivus, from elect-, past-participle stem of eligere "to pick out, choose" (see election). In U.S., in reference to school subjects studied at the student's choice, first recorded 1847. As a noun, from 1701.