Should I use 'other' or 'others' as an option item?
No matter if the option contains one or several members, the term to use is
other.
Firstly, this is the term most commonly used.
Also, the reason seems to be that the word 'other' here is not a noun but an adjective: it does not refer to 'the other' but the set of 'other options'. As other is not a noun here, it cannot be pluralized.