What does "una corda" refer to in piano playing?

"Una corda" refers specifically to the technique in piano playing where the performer plays with one of the hammers hitting only one string of a note, rather than the usual two or three strings that are typically struck when you play the piano. In this context, "una corda" translates from Italian to "one string," which is the essence of this technique.

When the una corda pedal is engaged, it shifts the action of the piano so that the hammer only strikes one string rather than the full set, resulting in a softer and more muted tone. This practice allows the pianist to achieve a delicate sound that contrasts with the full, richer timbre produced by playing all the strings. Therefore, the correct understanding of "una corda" centers on the concept of using just one string, which directly aligns with choice A.