Final answer:
In the poem "A Song for St. Cecelia's Day," the instrument that is a call to action is the violoncello. The instrument represents the power of music to evoke deep and passionate responses in listeners.
Step-by-step explanation:
In the poem "A Song for St. Cecelia's Day," the instrument that is a call to action is the violoncello. The poet describes how the sound of the violoncello stirs strong emotions in him, causing intense feelings in his belly and breast. The instrument represents the power of music to evoke deep and passionate responses in listeners.