For exponential functions y=ab^x, where a is the scale vertical stretch factor, b is the base, and x the exponent (also independent variable)
It is a know fact that if the scale factor (1/2 or 2 in the answer choices) is positive, the graph is increasing if the base (3 or 1/3) is greater than 1. This means that the base cannot be equal to 1/3. This eliminates choices 3 and 4.
Next, for a base greater than 1 (3>1), and the scale factor positive, it is a dilation (image gets bigger) if the scale factor is greater than 1. This eliminates the first choice where a=2.
This leaves us with the second choice where a=1/2, b=3, which is a shrink of the given exponential base function.
