OPTION A
A syntactic metalanguage is a notation for defining the syntax of a language by use of a number of rules. Each rule names part of the language (called a non-terminal symbol of the language) and then defines its possible forms. A terminal symbol of the language is an atom that cannot be split into smaller components of the language. A syntactic metalanguage is useful whenever a clear formal description and definition is required or we can say defining language for developing a formalism in which language definitions can be stated, is called a syntactic metalanguage.
A formal syntax definition has three distinct uses:
a) it names the various syntactic parts (i.e. non-terminal symbols) of the language;
b) it shows which sequences of symbols are valid sentences of the language;
c) it shows the syntactic structure of any sentence of the language.
for more details:-
https://www.iso.org/obp/ui/es/#iso:std:iso-iec:14977:ed-1:v1:en
