Which instrument was popularized during the classical period because of its responsive action and better gradual dynamics?

I believe the correct answer is piano.

In the art epoch of Classicism (19th century), the instrument which was popularized because of its responsive action and better gradual dynamics was piano. Because of its gradual dynamics, piano replaced harpsichord and became the favorite instrument of classical sonata.

photography had emerged as an elite art pursued by a few creative individuals for a few wealthy patrons. but now, said bauldelaire, it was succumbing to "the great industrial madness of our times." it was becoming a mass production industry that catered to middle-class tastes, including a taste for gadgetry. the stereoscope, the first 3d imaging system, had turned looking at photographs into parlor entertainment. a "thousand hungry eyes were bending over the peepholes of the stereoscope as though they were the attic-windows of the infinite.