In C, an identifier is a name used to identify a variable, function, or other element in a program. There are a few rules that must be followed when naming an identifier in C:

1. Identifiers must start with a letter or an underscore: Identifiers can start with a letter (either uppercase or lowercase) or an underscore (_).
2. Identifiers can contain letters, digits, and underscores: After the first character, identifiers can contain letters (either uppercase or lowercase), digits, and underscores.
3. Identifiers cannot contain spaces or special characters: Identifiers cannot contain spaces or special characters, such as punctuation marks or symbols.
4. Identifiers are case-sensitive: In C, identifiers are case-sensitive, which means that myVariable and myvariable are treated as different identifiers.
5. Identifiers cannot be the same as a keyword: In C, there are a number of reserved words, known as keywords, that have special meanings in the language. These keywords cannot be used as identifiers.

It is important to follow these rules when naming identifiers in C to ensure that your code is correct and can be compiled without errors. It is also a good practice to choose descriptive and meaningful names for identifiers to make your code easier to read and understand.