The high reached by the block when it approaches point C, depends of the potential energy at the beginning of the motion.
The potential energy is:
U = m·g·h
due to the path is frictionless, the energy of the system is constant. Then, while the block approaches to point C, all its energy becomes potential energy and this potential energy must be the same that the initial potential energy.
Due to the mass m and g are constant, and U must be the same, then, it is necessary that h, the height, is the same that at the beginning of the motion.
Hence, the hight is 4.5 m (the block does not reache point C) as the block approaches point C.