The correct answer is: A. Not all formal languages are context-free
A formal language is a set of strings of symbols that are formed according to certain rules. A context-free language is a type of formal language that can be described by a context-free grammar. A context-free grammar is a set of rules that specify how to generate strings of symbols from a set of non-terminal symbols.
Not all formal languages are context-free. For example, the language of all valid programs in a programming language is not context-free. This is because the rules of a programming language are
not always context-free. For example, the rule that a variable name must start with a letter is not context-free, because it depends on the context in which the variable name appears.The other options are incorrect. Option B is incorrect because there are formal languages that are not context-free. Option C is incorrect because natural languages are not formal languages. Option D is incorrect because natural languages are not context-free.