Final answer:
Vincenzo Bellini was the opera composer whose melodic style influenced Chopin's nocturnes, bringing a sense of lyrical beauty and expressiveness to the piano compositions of the Romantic era.
Step-by-step explanation:
The opera composer whose melodies influenced Frédéric Chopin's nocturne style was Vincenzo Bellini. Bellini was an Italian composer renowned for his flowing melodic lines, which had a substantial effect on Chopin's approach to melody in his nocturnes. Chopin greatly admired Bellini and his works, and the melodic elegance and emotional expressiveness found in Bellini's operas can be detected in Chopin's piano compositions. Despite living in the Romantic era, which produced many influential composers like Brahms, Tchaikovsky, and Schumann, the unique beauty of Bellini's melodies held a special place in influencing Chopin's development of the nocturne form, a genre that emphasizes lyrical melodies and a somewhat melancholic or reflective mood.