The novelist devotes so much time to avid descriptions of his characters’ clothes that the reader soon feels that such (i) __________ concerns, although worthy of attention, have superseded any more directly literary aims.
The novelist is concerned with the avid descriptions of his character's clothes and he devotes much time to it. The only word from the choice list that will correctly describe his concerns is sartorial.
Since the concerns are described as worthy of attention, we can rule out irrelevant and frivolous.
didactic refers to a method of instruction that is unidirectional from teacher to student.
syntactical refers to grammatical elements.
Both these are irrelevant and can be discarded.