Plug in some of the points of the graph into the equations to see if they are part of the graph.
Take the vertex (3,0)
0 = 1/2(3-3)
0 = 0 This is a true statement, thus (3,0) is a point on the equation y = 1/2(x-3) however this equation has the graph pointed up since all the y values would be positive. The graph we are looking at is pointing down. Thus A is wrong.
B and D both yield false statements when you plug that point into the equation.
Thus we are left with C and it makes sense because that graph will be pointing down like it is in the given graph.