The smallest possible unit of electromagnetic energy is the:

The smallest possible unit of electromagnetic energy is known as a photon. A photon is a quantum of electromagnetic radiation, which means it represents a discrete packet of energy associated with electromagnetic waves, such as visible light, X-rays, and gamma rays.

Photons have no mass and travel at the speed of light, making them fundamental to the interaction of light with matter. When we talk about X-rays in particular, understanding that they are made up of photons helps to explain how they can penetrate materials and produce images in medical imaging.

The other options, like ions, atoms, and molecules, represent different concepts. Ions refer to charged particles that have lost or gained electrons, atoms are the basic units of chemical elements composed of protons, neutrons, and electrons, and molecules consist of two or more atoms that are bonded together. None of these are units of electromagnetic energy, which is why photon is the correct answer for this question.