This is an anagram - look for an indicator word that suggests mixing or rearranging letters.
'Transposing' is the anagram indicator. You need to rearrange the letters from 'soprano' (abbreviated), 'flute', and 'line' (abbreviated). Look for single-letter abbreviations for 'soprano' and 'line'.
Answer: FULLEST | 'Transposing' indicates an anagram of S (soprano) + FLUTE + L (line) = FULLEST