The electromagnetic radiation with the smallest wavelength among the given options is Gamma rays.

Let's go through each option to understand more clearly:

Option A: X-rays

X-rays have very short wavelengths, typically in the range of 0.01 to 10 nanometers. They are used for medical imaging and other applications due to their ability to penetrate materials.

Option B: Gamma rays

Gamma rays have the smallest wavelengths of all electromagnetic radiation, often less than 0.01 nanometers (or 0.1 angstroms). They are highly energetic and are produced by nuclear reactions, radioactive decay, and other high-energy processes.

Option C: Ultraviolet rays

Ultraviolet (UV) rays have wavelengths ranging from about 10 to 400 nanometers. They are responsible for causing sunburns and are used in various scientific and industrial applications.

Option D: Microwaves

Microwaves have much longer wavelengths, typically ranging from 1 millimeter to 1 meter. They are commonly used in communication technologies and for heating food in microwave ovens.

Based on these explanations, the correct answer is:

Option B: Gamma rays