Which of the following best describes an electrolyte?

The best description of an electrolyte is a liquid that contains ions. Electrolytes are substances that dissociate into ions when dissolved in a solvent, typically water. This ionic dissociation allows the solution to conduct electricity. Common examples of electrolytes include salt solutions, such as sodium chloride in water.

When it comes to the other choices, a solid containing ions does not necessarily conduct electricity unless it is melted or dissolved to allow the ions to move freely. Gases that react with electrodes do not typically represent the behavior expected of electrolytes, as conductivity is primarily associated with liquid or molten states. Lastly, a compound that conducts electricity in vapor form is not an accurate description of electrolytes, as gases generally do not conduct electricity in the same way that liquids do. Therefore, the distinguishing feature of an electrolyte being a liquid containing ions highlights its ability to facilitate electrical conduction effectively.