Answer
(b)
Work Step by Step
The graph of $3^x-1$ is just the graph of $3^x$ shifted vertically down by one unit which is (b). Let's find out the crucial limits to get some more insight.
$\lim\limits_{x \to -\infty}3^{x}-1 = -1$
$\lim\limits_{x \to \infty}3^{x}-1=\infty$
both of which are same as that of the graph in (b)
