Which of the following defines a solvent?

A solvent is defined as a liquid that has the capability to dissolve another substance, which is known as a solute, to form a solution. This definition captures the essence of what a solvent does in chemical processes, where it typically facilitates solutions by surrounding and interacting with solute particles, thus breaking them apart and distributing them throughout the liquid.

Other options do not align with this definition. For instance, a solid providing structural integrity refers to a different concept, associated more with materials and their physical properties rather than solvency. A mixture of gases does not pertain to the concept of a solvent, as it does not involve dissolving a substance within a liquid state. Finally, a chemical reaction producing heat describes a thermodynamic process, which is separate from the concept of solvents and their role in solutions.