Producers of jazz albums tend to back artists that they expect will ga

meaning wise,
only A and C makes sense.

A) that [url]they expect[/url](Additional info. read without this) will gain popularity in the key European market, and who are
C)they expect to gain popularity in the key European market, and who do(Replace do with gain)

To me A makes more sense that C because,
I) Artist someone expects(future) will gain popularity. Future tense is used properly.
II) "Who are" says that artists who are gaining popularity.

So the overall sentence is XYZ tend to back 1) artists who will gain popularity and 2) who are gaining popularity

Whereas in C, I am skeptical about the usage of WHo do.
The sentence is trying to convey that
XYZ tend to back 1)artist sthey expect to gain popularity in the key European market, and 2) who gain popularity(Simple present tense. Trying to convey that artists who always gain popularity).

Gaining popularity is not a point action but is a cumulative action and hence I suspect "Artists who gain' isnt a logical way to communicate.

Bunuel Can you comment on how A is inferior to C?