Explanation:

A 'programming language' is a formal language comprising a set of instructions used to produce various kinds of output. It is a set of rules that allow a programmer to specify the operations a computer should perform. Option (a) 'Procedural language' is a *type* of programming language, focusing on a sequence of statements to change program state, but 'programming language' is the overarching term. Option (b) 'Structures' refers to data organization, not a set of rules for operations. Option (c) 'Natural language' refers to human languages, which computers do not directly understand for executing operations. Option (d) 'Command language' is used for issuing specific commands, but a programming language provides a much broader and more complex set of rules for defining operations. Therefore, 'programming language' is the most accurate and comprehensive term.