B. (0, +infinity)
When it is asked to know (b×a)(x), it is being asked to know what are the Y coordinates that are common in both graphs. When it a multiplication think of "and" - what are the Y that are on one graph and simultaneously on the other graph.
Design the graph of both functions on a calculator, or online, wherever. Then, look for when both functions have coordinates. All of them that are common to both graphs are the domain of (b×a)(x).
