Explanation:

In the context of formal language theory, a Context-Free Grammar (CFG) is a formal grammar consisting of the following elements:

a) A set of terminal symbols:
Terminal symbols, also known as terminal alphabets or simply terminals, are the basic building blocks or atomic elements of a language. They are the smallest units that cannot be further divided. Examples of terminal symbols include letters, digits, punctuation marks, and special characters. In a CFG, the set of terminal symbols is denoted by Σ.

b) A set of non-terminal symbols:
Non-terminal symbols, also known as non-terminals or variables, are symbols that can be replaced or expanded into a sequence of symbols. They represent different categories or types of elements in the language. Non-terminal symbols are used to define the structure or syntax of the language. In a CFG, the set of non-terminal symbols is denoted by N.

c) A set of productions:
Productions, also known as rewrite rules or production rules, define the rules or transformations that can be applied to the non-terminal symbols. Each production consists of a non-terminal symbol called the left-hand side (LHS) and a sequence of terminals and/or non-terminals called the right-hand side (RHS). The application of a production replaces the LHS with the RHS. Productions are used to generate the valid strings or sentences of the language. In a CFG, the set of productions is denoted by P.

d) All of the mentioned:
The correct answer is option 'D' because all of the mentioned elements (terminal symbols, non-terminal symbols, and productions) are essential components of a CFG. Each element plays a crucial role in defining the language and its syntactic structure. Terminal symbols provide the basic vocabulary or symbols of the language, non-terminal symbols define the structure or syntax of the language, and productions specify the rules for generating valid sentences. Together, these elements form the foundation of a CFG and allow for the generation and analysis of context-free languages.