Which one of the following represents a valid variable name?
a. 2a
b. $2
c. d+4
d. d 4

b. $2

Explanation:

a. 2A is not a valid variable because variable names cannot begin with a number.

b. $2 is valid because variable names can begin with $ or _ characters.

c. d+4 is not a valid variable because variable names can only contain letters, numbers, underscores, or dollar signs. (+) sign is invalid.

d. d 4 is not a valid variable because variable names cannot contain spaces.

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