The answers are C and F.

The sentence says that although Ferneyhough’s individual compositions are highly distinctive, musicologists struggle to trace the overall development of his body of work because he did not keep organized records or dates. So the blank must convey the idea of something difficult to pin down or clearly track.

A. inchoate
Inchoate means undeveloped or not fully formed. The sentence does not suggest the oeuvre is incomplete or immature; the difficulty lies in tracing its development.

B. inscrutable
Inscrutable means impossible to understand. While the oeuvre may be difficult to organize chronologically, the sentence does not suggest the music itself is fundamentally incomprehensible.

C. diffuse
Diffuse means scattered, lacking clear organization, or spread out. This fits well because the lack of systematic records and dates makes the body of work seem difficult to organize coherently.

D. refractory
Refractory means stubbornly resistant or difficult to control. This word is generally used for people, substances, or systems resisting control, not for an oeuvre that is difficult to trace historically.

E. derivative
Derivative means unoriginal or imitative. This directly contradicts the description of the compositions as “strikingly idiosyncratic.”

F. elusive
Elusive means difficult to grasp, define, or pin down. This fits perfectly because the lack of dates and revisions makes the development of the oeuvre hard to trace clearly.