1. The problem asks to identify which of the given functions is an exponential function. 2. An exponential function has the form $$y = a^x$$ where the variable $$x$$ is in the exponent and $$a$$ is a positive constant not equal to 1. 3. The first function is $$y = 2x + 4$$. Here, $$x$$ is multiplied by 2 and then 4 is added. This is a linear function, not exponential. 4. The second function is $$y = 3^x$$. Here, $$x$$ is the exponent of the base 3, which fits the definition of an exponential function. 5. Therefore, the exponential function among the given options is $$y = 3^x$$. Final answer: $$y = 3^x$$