Explanation

‘Neither fish nor fowl’ describes a person or thing that does not fit into any recognized category — something hard to classify or define. This is the dictionary sense of the idiom (not belonging to any particular type or group). Option (b) (‘neither this or that’) is a vague literal paraphrase that misses the idiom’s specific sense of unclassifiability. Options (c) and (d) are unrelated. Hence ‘Difficult to describe or classify’ is the most appropriate meaning.