What makes an exponential exponential is that the exponent is the variable. So unlike polynomials that have [tex]x^2[/tex] and [tex]x^5[/tex] in them, polynomials are switched and have the base as a fixed number and the variable as the exponent: [tex]y=b^x[/tex].
The last answer, y equals b to the power of x, is the best, from this list of options.
