Which option is a phase of matter?

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A phase of matter is one of the distinct forms matter can take (solid, liquid, gas, and the energetic fourth state called plasma). Plasma forms when enough energy is added to a gas that electrons are knocked free from atoms, creating a highly energetic mix of free electrons and positive ions. This ionized state conducts electricity and responds to magnetic fields, and you encounter it in stars, lightning, and many lighting displays. Among the options, plasma matches the idea of a phase because it is a unique state of matter with properties that set it apart from solid, liquid, and gas. Foam isn’t a single phase—it's gas bubbles dispersed in a liquid or solid, making it a multi-phase system. An alloy is a solid mixture of metals, not a fundamental state of matter. A solution is a homogeneous mixture (one phase) but not one of the basic states of matter.