Answer 1 or 2 will do. The multiple functions you list make up a Set of functions. You have found that only one set of functions make a solution.

"The set of functions f(x) = 1, g(x) = 2x and h(x) = x + 2 is the only solution to the given problem."

It is common to use and in place of the last comma separating an enumerated list. You may even use both as in "g(x) = 2x, and h(x) = x + 2" according to Warner's English Grammar.

Answer 1 or 2 will do but the following should be added. The multiple functions you list make up a Set of functions. You have found that only one set of functions make a solution.

"The set of functions f(x) = 1, g(x) = 2x and h(x) = x + 2 is the only solution to the given problem."

It is common to use and in place of the last comma separating an enumerated list. You may even use both as in "g(x) = 2x, and h(x) = x + 2" according to Warner's English Grammar.