The functions f(x) = 1, g(x) = 2x and h(x) = x + 2 are the only solutions to the given problem.

This is the only correct version.

"f(x) = 1, g(x) = 2x and h(x) = x + 2" stand in apposition to "functions". As such they do not influence the plurality of the subject which is "functions."

Functions is plural therefore the verb is "are".

The functions f(x) = 1, g(x) = 2x and h(x) = x + 2 are the only solutions to the given problem.

This is the only correct version.

"f(x) = 1, g(x) = 2x and h(x) = x + 2" stand in apposition to "functions". As such they do not influence the plurality of the subject which is "functions."

Functions is plural therefore the verb is "are".