An identifier in C

The correct answer is D. All of the above.

An identifier in C is a name of a thing such as variable, function, structure, union, or label. It is made up of letters, numerals, and the underscore. It can contain both uppercase and lowercase letters.

Here are some examples of identifiers in C:

• x
• y
• z
• a
• b
• c
• d
• e
• f
• g
• h
• i
• j
• k
• l
• m
• n
• o
• p
• q
• r
• s
• t
• u
• v
• w
• x
• y
• z
• 0
• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 8
• 9
• _
• A
• B
• C
• D
• E
• F
• G
• H
• I
• J
• K
• L
• M
• N
• O
• P
• Q
• R
• S
• T
• U
• V
• W
• X
• Y
• Z

The first character of an identifier must be a letter or an underscore. The remaining characters can be letters, numerals, or underscores.

Identifiers are case-sensitive. For example, x and X are different identifiers.

Identifiers cannot be keywords. Keywords are reserved words that have special meanings in the C language. For example, int is a keyword that cannot be used as an identifier.