According to Einstein's theory of relativity, when a body moves with a velocity comparable to the velocity of light, its length increases but mass decreases. This phenomenon is known as length contraction and mass dilation. Let's understand this in detail:

1. Length Contraction:
When an object moves at a velocity close to the speed of light, its length in the direction of motion appears to contract or shorten when observed from a stationary frame of reference. This means that the length of the object measured by an observer in motion will be smaller compared to the length measured by an observer at rest. This effect becomes more significant as the velocity approaches the speed of light.

2. Mass Dilation:
According to the theory of relativity, as an object moves at a velocity close to the speed of light, its mass increases. This effect is known as mass dilation or relativistic mass. The mass of an object measured by an observer in motion will be greater than the mass measured by an observer at rest. The increase in mass is a result of the kinetic energy of the object, which becomes significant as the velocity approaches the speed of light.

3. Time Dilation:
In addition to length contraction and mass dilation, Einstein's theory of relativity also predicts time dilation. This means that time appears to run slower for an object in motion compared to an object at rest. As the velocity of an object approaches the speed of light, time dilation becomes more pronounced.

The phenomenon of length contraction, mass dilation, and time dilation are consequences of the theory of relativity and have been experimentally verified. These effects have significant implications in the field of physics, especially in understanding the behavior of objects that move at high velocities, such as particles in accelerators or celestial objects traveling at relativistic speeds.

In conclusion, according to Einstein's theory of relativity, when a body moves with a velocity comparable to the velocity of light, its length increases but mass decreases. These effects, known as length contraction and mass dilation, are fundamental concepts in understanding the behavior of objects in motion at relativistic speeds.

Correct ans is d