Here is my views for this CONCLUSION family CR problem.
Once I see that Q stem is asking for conclusion, I rushed to the option which can answer "why" from the given information in argument.
So lets have a look.
(A) Sales of compact discs containing media other than music are sustaining retailers. +Why?
No such information is provided for retailers' sustainability for media other than music.
(B) Consumers are turning increasingly to digital media to purchase music. +Why?
Why consumers are turning to other forms of music! Not answered by given info.
(C) The viability of the compact disc format was challenged in the last five years. +Why?
See the trend of music format. Sales of CDs are declining. But music records are rising in last 3 years. Therefore, CD format is not alluring to listeners.
(D) Record companies have invested in new media without retailer consent. +why?
First of all, there is no point of investment in argument. Consent! It is also not mentioned.
(E) The music recorded and sold on compact discs has declined in marketability.+why?
Close to answer but this is tweaked premise. So it is not a conclusion.
C is winning choice.
Any other explanations are still awaited.