Such a hard question ...
However, IMO
AOf millions of contemporary musicians playing in Britain today, professional songwriters among them, many can't play a note.a. Of the millions of contemporary musicians playing in Britain today, many professional songwriters can't play a note -->
Precise meaning, correct use of millions of con.... today -->the best.
The intended meaning of the sentence is that millions of contemporary musicians playing in Britain today include many musicians who can't play a note, and among such musicians who can't play a note, suprisingly, there are many professional ones b. Many professional songwriters, out of millions of contemporary musicians playing in Britain today, can't play a note -->
this illogically indicates that many professional songwriters are only musicians who can't play a note while the rest of millions of contemporary ... britain today can. The use of out of, meaning a particular selection of a group, changes the intended meaning to a wrong meaning of the sentence c. Professional songwriters are among millions of contemporary musicians playing in Britain today who can't play a note -->
this choice changes the original meaning of the sentence to a ridiculous meaning d. Of millions of contemporary musicians playing in Britain today, professional songwriters among them, many can't play a note -->
among them seems redundant with of millions ... today e. Of the millions of contemporary musicians playing in Britain today, professional songwriters among them, many can't play a note -->
same error as (D)