Final answer:
Relativistic quantum mechanics describes small objects traveling at high speeds or experiencing strong gravitational fields, combining the principles of both relativity and quantum mechanics.
Step-by-step explanation:
The subfield of physics that deals with small objects traveling at high speeds or experiencing a strong gravitational field is known as relativistic quantum mechanics. This combines the principles of both relativity and quantum mechanics. Relativity describes objects moving at significant fractions of the speed of light or within strong gravitational fields, while quantum mechanics describes the behavior of atomic and subatomic particles. Therefore, the correct answer to the question posed by the student would be a combination of both quantum mechanics and relativity, rather than a specific option listed initially.
Classical mechanics, such as explained by Newton's laws, is not sufficient to describe such scenarios. For example, the motion of a plane through the sky (classical level) can be described with classical physics, but the strength of gravity within a black hole (strong gravitational field) and the collisions of subatomic particles (high speeds at a very small scale) need relativistic and quantum descriptions, respectively.