The graph shows the relationship between substrate concentration and reaction rate in an enzyme-catalysed reaction - HSC - SSCE Biology - Question 5 - 2016 - Paper 1

At point X, the reaction rate reaches a maximum despite increasing substrate concentration. This plateau indicates that all the enzyme active sites are occupied by substrate molecules, leading to a state of saturation. At this point, adding more substrate will not increase the reaction rate because there are no available active sites for the enzyme to bind. Therefore, the correct answer is (D) All the enzyme active sites are being occupied.