The correct option is D All the particles of the medium, except those at nodes, execute simple harmonic motion about their mean positions.
A stationary wave has the following characteristics:
The maximum velocity is different at different points. Its value is zero at the nodes and gradually increases towards antinodes. All the particles attain their maximum velocities simultaneously when they cross their mean position.
Thus, the disturbance doesnt move in any direction. The conditions of crests and troughs merely appear and disappear in fixed positions in fixed intervals of time.
Particles belonging to a particular segment (crest or trough) vibrate in the same phase but the particles in the neighbouring segments vibrate in opposite phases.
It is clear that, except the particles at nodes, all the particles of the medium execute SHM about their mean position.
Thus, all the options are correct answers.