**Explanation:**

Vibration refers to the rapid back-and-forth movement of an object or a medium. It is characterized by its frequency, amplitude, and wavelength. In most cases, vibrations of objects are not visible to the naked eye, and the correct answer is option 'C' - amplitude is very small. Here's why:

**1. Amplitude:**
The amplitude of a vibration refers to the maximum displacement of an object from its rest position. In other words, it represents the distance between the object's extreme positions during vibration. When the amplitude is very small, the object moves back and forth over a very small distance. This small movement is often not visible to the human eye, making the vibrations appear invisible.

**2. Perception of Motion:**
The human eye is capable of perceiving motion when the displacement of an object or a medium is significant enough to be noticeable. If the amplitude of an object's vibration is very small, the movement is not substantial enough to be detected visually. Therefore, even though the object may be vibrating, it appears still to our eyes.

**3. Wavelength and Frequency:**
The wavelength and frequency of vibrations do not directly affect their visibility. Wavelength refers to the distance between two consecutive points in a vibration that are in phase (e.g., two crests or two troughs). Frequency, on the other hand, is the number of complete vibrations that occur in a given time period. While wavelength and frequency play a role in the characteristics of a vibration, they do not determine its visibility.

**4. Examples:**
Many objects around us vibrate, but we are unable to see their vibrations due to the small amplitude. For example:

- When a guitar string is plucked, it vibrates at a frequency that produces sound. However, the amplitude of the vibration is tiny, and we can only hear the sound produced, not see the string vibrating.
- Similarly, the vibrations of molecules in the air create sound waves, but we cannot see these vibrations as they have a small amplitude.
- Other examples include the vibrations of atoms, the oscillations of pendulums, or the shaking of a mobile phone on vibrate mode. While these vibrations occur, they are not visible to the naked eye due to their small amplitudes.

In conclusion, the main reason vibrations of objects are not visible is that their amplitudes are very small. The human eye requires a significant displacement to perceive motion, and when the amplitude is tiny, the vibrations appear invisible.

BECAUSE LARGE AMPLITUDE LARGE SOUND SMALL AMPLITUDE SMALL SOUND
SO THE ANSWER IS OPTION C