MS-DOS originally single user, multi-user support retrofitted
XENIX always been multi-user
OS/2 Always been Single user, while later versions added multi-user support but when multi-user support added then the name of OS was changed to 'OS/2 warp' and 'Windows NT;
based on these safest choice is option A, 'a only'. Because option B,C,D has 'b. XENIX' which has been a multi-user OS from start.
OS/2 does not support multi user
visit above page scroll down to limitations
XENIX Supports multi user feature
MS-DOS single user os though some latter versions does tried to implement multi user support but it is considered single user os as multi user support were not there in it originally and was retro fitted.