Like terms that have the same with each variable raised to the same exponent.

operation(s)
variable(s)
expression(s)
exponent(s)

From the given options, the correct word that comes in the blank is variables.
Because like terms are the terms that have the same variables with each variable raised to the same exponent. 
Unlike terms are the opposite of like terms, as they do not have the same variables and exponents or powers.
like terms and unlike terms are used in algebra.