The correct answer is option 'C' - amplitude.

Waves are a common phenomenon that we encounter in our daily lives. They are characterized by the transfer of energy through a medium or space. Waves can be found in various forms, such as sound waves, water waves, light waves, and electromagnetic waves. These waves exhibit certain properties, including amplitude, wavelength, frequency, and velocity.

Amplitude is a physical quantity that measures the maximum displacement or distance from the equilibrium position of a wave. It represents the magnitude or intensity of the wave. In other words, amplitude determines the height or strength of the wave. For example, in the case of a water wave, the amplitude represents the height of the crest or the depth of the trough.

Explanation:

Oscillation is the repetitive motion or movement of a wave. It refers to the back-and-forth motion of particles or disturbances caused by the wave. When a wave undergoes oscillation, it means that the particles of the medium are vibrating or moving periodically around their equilibrium positions.

Amplitude is directly related to oscillation. As the wave oscillates, the particles move from the equilibrium position to a maximum displacement, which is determined by the amplitude. The particles then return to their original position before moving in the opposite direction to reach the maximum negative displacement.

In other words, the amplitude determines the range of motion or the extent of the back-and-forth movement of the particles. It represents the maximum displacement of the particles from their equilibrium position. As the wave propagates through the medium, the particles continue to oscillate around their equilibrium positions, creating a repetitive pattern of motion.

Therefore, the physical quantity that oscillates in most waves is the amplitude. It plays a crucial role in determining the intensity, loudness, or brightness of the wave. By adjusting the amplitude, we can control the strength or magnitude of the wave, leading to variations in its effects and perception.