During the Romantic period, one significant change made to the piano was the addition of the sustained pedal (also known as the "damper pedal"). This innovation allowed for greater expressiveness and the ability to sustain notes longer, which was highly valued by composers and performers of that era. Other changes to the piano during this time included improvements to the action and the use of a sturdier cast iron frame, which allowed for increased tension in the strings and greater dynamic range, as well as expansions in range and tone.
Thus, the correct answer from your options is that the sustained pedal was added.