(i) Keyword: Keywords are special words in C programming which have their own predefined meaning. The functions and meanings of these words cannot be altered. Some keywords in C Programming are if, while, for, do, etc..
(ii) Identifier: Identifiers are user-defined names of variables, functions and arrays. It comprises of combination of letters and digits. Example int age1; float height_in_feet; Here, age1 is an identifier of integer data type. Similarly height_feet is also an identifier but of floating integer data type,
(iii) Variable: A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in C has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable. Example: add, a, name
Constants refer to fixed values that the program may not change during its execution. These fixed values are also called literals. Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are enumeration constants as well.