a. y = (1/2)x^2:
This function is not an exponential growth function. It is a quadratic function, as indicated by the presence of the exponent 2 in the x term. Quadratic functions have a "U" or "n" shaped graph, which does not exhibit exponential growth.
b. y = 2(1/3)^x:
This function does represent exponential growth. It is an exponential function with a base of 1/3 raised to the power of x. As x increases, the value of (1/3)^x becomes smaller, and when multiplied by 2, the overall function value decreases. This behavior is typical of exponential decay, not growth.
Therefore, among the given options, none of the provided functions represent exponential growth.