For example, there is a course (say French course), for students in a college. The students can take it, but they don't have to. Someone, who is not a student in that college, thinks that this course is optional. But he saw, on a student's facebook wall, that "I have a french course." So he wants to ask this student that if this course is optional.
Of course he can just ask:
Is this course elective/optional?
But the above is just an example to describe the words or the expressions that I need:
An expression that says
One can choose to do a particular thing by his own willing.
There is a constrain for such expressions:
If a thing breaks the law or the rules (of a society, company, school...), then we can't use this expression for such a thing.
I am not sure if
... can do...by ...own willing is a common expression for this. Furthermore, is there any other/short way to express this?