Which word describes someone who is lacking the ability to speak?

The term that describes someone who lacks the ability to speak is "mute." This word specifically refers to an individual who is unable to produce speech sounds, either due to a physical inability or a psychological condition. The usage of "mute" is rooted in both medical and social contexts, indicating a defined absence of speech rather than a person's capacity to understand or convey ideas through other means.

In the context of the other options, "vocal" refers to someone who can speak or produce sound, "articulate" describes a person who speaks fluently and coherently, and "expressive" implies an ability to convey thoughts and feelings, often through speech or other forms of communication. Each of these terms highlights different aspects of communication abilities, which underscores why "mute" is the most accurate choice for describing someone who cannot speak.