The correct option is B Serenade
One word for the given phrase 'Music sung or played at night below a person's window' is option A 'Serenade'. Option B 'Sonnet' is a fourteen line rhyme with a specific rhyme scheme. Option C 'Lyric' is the words of a song. Option D 'Primo' is the right hand part in a piano duet. So, options B, C and D are incorrect and the correct option is Option A, Serenade.