Some operating systems, like Linux and macOS, only allow:

Question $3$Answer

a$.$

PTHREAD SCOPE PROCESS for user$-$level thread scheduling.

b$.$

Specifying the scheduling policy $($e$.$g$.,$ priority, round$-$robin$)$ for user$-$level threads.

c$.$

The use of SCS $($PTHREAD SCOPE SYSTEM$)$ for thread scheduling.

d$.$

A single user$-$level thread per process.