Final answer:
In 'A Song for St. Cecelia's Day,' the violoncello is depicted as the instrument that embodies a bundle of intense emotion, specifically resembling a young man's heartfelt complaint.
Step-by-step explanation:
In "A Song for St. Cecelia's Day," the instrument described as a bundle of intense emotion is the violoncello. This is indicated by the line that reads, 'I hear the violoncello, ('tis the young man's heart's complaint,)'. The poem equates the sound of the violoncello with the deep and tumultuous emotions that surge from the heart of a young man, likely in love or dealing with sorrow. The passage suggests that the rich, deep tones of the cello are capable of expressing a wide range of powerful human emotions akin to a young man's heartache.