optional

adj. /ˈɑːp.ʃən.əl/ B2

选修的；非强制的

Available to be chosen but not mandatory or compulsory.

仙境例句

Music was optional at the sea school, but every young turtle wanted to learn to sing.

在海底学校，音乐是选修课，可每只小海龟都想学唱歌。