Here s my point of view:
BF1- But most musicians who record albums after the age of 35 started to record music only after they turned 30.
Sounds like a fact or premise or could be used to support an argument
BF2- many musicians over the age of 35 produce fewer albums because they have been making music for too long to write compelling new music.
Since it is prefaced by - So instead of arguing that old age reduces creativity, it can instead be suggested that
This is the authors conclusion.
A- Each provides support for the primary explanation for the phenomenon described in the passage.
BF1 could be a support but BF2 is not- eliminate
B- The first is a rebuttal of an assertion made by the argument; the second is that assertion.
BF1 is a statement not a rebuttal, and neither is it asserting against BF2- eliminate
C- The first provides evidence in support of an explanation that the argument challenges; the second is that explanation.
BF1 is a support, it is challenging an argument made in the opening lines of the statement, so here is where one can get confused. BF2 is not the explanation that BF1 is challenging, instead it is used to support BF2- eliminate
D- The first offers evidence in support of an explanation with which the argument agrees; the second is that explanation.
BF1 offers support to an explanation is correct, BF2 is the explanation that the argument agrees with, hence the entire statement becomes true.
E-The first offers evidence in support of an explanation that the argument challenges; the second is an explanation with which the argument agrees.
Similar to C.- eliminate
Although the argument is challenging the line- “Some have argued that this phenomenon is the result of older age diminishing creative processes.” When you look at options CDE, C and E make us believe that the author is arguing against his own conclusion which is false.