The president of NFX Media Corporation is a ____ professional soccer player.

A) once
B) previous
C) former (correct)
D) past

Why can't I choose B)? Is that because of the collocation? Please advise.

Yes, I suppose the collocation makes C the choice. I could choose D also; a poorly-conceived question, to my mind.
Bill Clinton is a former president of the USA. Here we are talking about Clinton only and what he used to do. We would say previous only when speaking in relation to George W Bush; previous means the one who preceded him. I used to go rowing, so I am a former oarsman. We're not speaking about somebody who took my place in the boat.
Students: Are you brave enough to let our tutors analyse your pronunciation?
I see. And it seems like we often use "former" or "ex- "to talk about someone with a position. Is that right?

Students: We have free audio pronunciation exercises.