According to special theory of relativity the mass of a particle A. Increases with increase in velocity with respect to an observer. B. Decreases with increase in velocity. C. Decreases with decrease in velocity. D. Is independent of its velocity.

The correct answer is A. Increases with increase in velocity with respect to an observer.

According to special relativity, the mass of a particle increases

with increase in velocity with respect to an observer. This is because the faster a particle moves, the more energy it has, and the more energy it has, the more mass it has. This is known as the mass-energy equivalence, which is expressed by the equation $E = mc^2$.

Option B is incorrect because the mass of a particle does not decrease with increase in velocity. Option C is incorrect because the mass of a particle does not decrease with decrease in velocity. Option D is incorrect because the mass of a particle is not independent of its velocity.