The set of rules that defines the combination of symbols that are considered to be correctly structured statements or expressions in that language is called...

Every programming language has a set of grammar rules that determine which combinations of symbols, keywords, and punctuation form a valid statement. This set of rules is called the syntax of the language.

Syntax defines the correct structure of instructions, for example the required order of keywords, where semicolons, brackets, or indentation must appear, and how expressions must be written. A statement that breaks these rules is described as having a syntax error, and the compiler or interpreter will reject it before it can even check whether the logic is correct.

An instruction is a single command the computer executes, a program is a complete set of instructions that solves a task, and a domain refers to a category of values or a network naming concept, none of these describe the rules governing correctly structured statements, syntax does.

Exam tip: whenever a question describes "rules for correctly structured statements or expressions" in a language, it is describing syntax, as opposed to semantics (which is about meaning) or logic (which is about the correctness of the reasoning).