Sentence improvement – idiom and collocation: prefer the fixed phrase 'harp upon/on one string only' to express tedious repetition; correct the noun 'string' and appropriate preposition.

Given sentence
'He should move on to the next point, and not harp one sting only.'

Concept/Approach
The idiom is 'harp on/upon one string' (or 'the same string') meaning to dwell tediously on one point. Fix both the preposition and the misspelling 'sting' → 'string'.

Step-by-step correction
Use the full idiom: '… and not harp upon one string only.'

Why other options are incorrect
'harp on string only' → missing article; sounds awkward.'harp only one string' → wrong structure (missing preposition).'No improvement' → keeps two errors.

Final Answer
harp upon one string only