Lexemes are the basic units of a computer program. They are the smallest meaningful units of code. In programming languages, lexemes are categorized into different types based on their functionality and purpose. The given options are:

a) Identifiers:
- Identifiers are names given to variables, functions, or other entities in a program. They are user-defined and used to uniquely identify these entities. Examples of identifiers include variable names like "count", "sum", or function names like "calculateTotal", "printMessage", etc.
- Identifiers are considered lexemes because they are basic units of code and play a crucial role in program execution.

b) Constants:
- Constants are fixed values that do not change during program execution. They are used to store data that remains constant throughout the program. Examples of constants include numeric constants like 10, 3.14, or string constants like "Hello", "World", etc.
- Constants are also considered lexemes because they are basic units of code and hold important data for program execution.

c) Keywords:
- Keywords are reserved words in a programming language that have predefined meanings and cannot be used as identifiers. They are used to define the syntax and structure of a program. Examples of keywords include "if", "else", "for", "while", etc.
- Keywords are also considered lexemes because they are basic units of code and have a specific role in program execution.

d) All of the mentioned:
- This option states that all the given options (identifiers, constants, and keywords) are lexemes. It is the correct answer because all three options are indeed lexemes.

In summary, lexemes are the basic units of a program, and in this case, the correct answer is option 'D' which includes all the mentioned options (identifiers, constants, and keywords). Identifiers are names given to program entities, constants are fixed values, and keywords are reserved words with predefined meanings. All three play a crucial role in program execution and are considered lexemes.