I think its B. My reasoning is that the philsophers ae essentially stating the caveats associated with experimental evidence of their theories. Meaning, they really dont believe its important to prove their theory.

my FA is C (Please provide reasoning with your answers - STOP the spirit of this forum getting under threat!)

My reasons:
A) Any particular scientific theory can be consistently retained, even in the face of apparently incompatible evidence, if we are willing to give up certain other scientific beliefs.
<B> "taking for granted such scientific beliefs" could mean taking for granted a set of universally accepted beliefs - this option goes too far </B>

B) Experimental evidence is really irrelevant to scientific theorizing.
<B> Eliminate this option as evidence could be relavant to scientific theorizing which is based upon a set of established scientific beliefs</B>

C) Experimental evidence is more relevant to the testing of scientific theories than to their initial formulation.
<B> I like the use of the word "more" - very GMATish. Also, this option concisely puts the intended summary of the argument </B>

D) Experimental evidence is more relevant to the initial formulation of scientific theories than to their testing.
<B> completely negates whatever is stated in the argument</B>

E) The best scientific theories are those which are formulated in such a way as to be subject to conclusive experimental refutation.
<B> out of scope of the argument as the author is not trying to compare scientific theories</B>