What does "tre corde" instruct the player to do on a piano?

The instruction "tre corde" translates to "three strings" in Italian and is used in piano music to indicate that the player should release the una corda pedal. The una corda pedal softens the sound by shifting the action so that the hammer strikes only one string (or two strings, depending on the design of the piano) instead of the usual three for each note. Therefore, when "tre corde" is indicated, it signals the performer to return to the normal position of the action where the hammers will strike the three strings, producing a fuller sound.

In the context of the options provided, releasing the left pedal aligns perfectly with this definition since the left pedal is typically the una corda pedal on a piano. Thus, this instruction tells the musician to return to the standard tone of the instrument, moving from a softer to a richer sound.