The correct answer is
f(x)=1/3(x+1)(x-2)(x-1)(x+5).
Looking at the graph, we can see the solutions (otherwise known as zeroes) of the function are at -5, -1, 1 and 2. This gives us the factors (x+5), (x+1), (x-1) and (x-2). This makes the last function the only possible one.
