"depending on preference," hence "that may be done or not done according to one's choice," 1765, from option + -al (1).
Entries linking to optional
option n.
c. 1600, "action of choosing;" 1630s, "power or liberty of choosing," from French option (Old French opcion), from Latin optionem (nominative optio) "choice, free choice, liberty to choose," from optare "to desire, pray for, choose," which is of uncertain origin. De Vaan derives it from Proto-Italic *opeje- "to choose, grab," from PIE *hopeie- "to choose, grab," and compares Hittite epp/app- "to take, grab," Sanskrit apa, Avestan apa "has reached."
The meaning "thing that may be chosen" is attested from 1885. The commercial transaction sense of "privilege secured by payment of a premium (on a stock or a certain produce at a specified time and at a specified price)" is recorded from 1755 (the verb in this sense is attested by 1880 in American English). As a North American football play in which the back may either pass the ball or run with it, it is recorded by 1953.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).