1. Identify the phenomenon

When a metal target is bombarded by high-energy electrons, the electrons undergo rapid deceleration on striking the target.

2. Name of the radiation produced

This sudden deceleration of fast electrons produces bremsstrahlung (braking radiation), which lies in the X-ray region of the electromagnetic spectrum.

Also, if the incoming electrons knock out inner-shell electrons of the metal atoms, characteristic X-rays are emitted when higher-level electrons fill those vacancies.

3. Evaluate the options

• A: Infrared rays — Incorrect. These are thermal radiations, not the primary radiation produced in this process.
• B: Radio Waves — Incorrect. These are much lower frequency waves and are not produced by electron bombardment of a metal target in this context.
• C: Microwaves — Incorrect. These are also low-frequency electromagnetic waves, not the result here.
• D: X-rays — Correct. High-speed electrons striking a metal target produce X-rays.

4. Final answer

Therefore, the waves emitted are:

$\boxed{\text{X-rays}}$