Which of the following best describes 'Lexis' in the context of programming languages?

In the context of programming languages, 'Lexis' refers specifically to the vocabulary or set of symbols used in the language. This includes keywords, identifiers, operators, and other elements that a programmer can use to write code. Lexis is a crucial consideration when designing a programming language because it determines the range of expressions and statements that can be formed in that language.

While the other options mention important concepts related to programming languages, they do not accurately define 'Lexis'. A set of rules pertains more to grammar and syntax rather than being focused solely on vocabulary. A set of commands suggests operational aspects, which are about what the language can do rather than the words used to express those commands. A syntax framework deals with the structure and arrangement of code elements, which is separate from the vocabulary itself. Therefore, describing 'Lexis' as the vocabulary of the language is the most appropriate characterization in this context.