The correct answer is A. Freyssinet system.
The Freyssinet system is a method of prestressing concrete using high tensile alloy steel bars (silica manganese steel). The bars are stressed and then embedded in the concrete, which helps to prevent cracking and improve the strength of the structure.
The Magnel-Blaton system is a method of prestressing concrete using wires or cables. The wires or cables are stressed and then embedded in the concrete, which helps to prevent cracking and improve the strength of the structure.
The C.C.L. standard system is a method of prestressing concrete using unbonded tendons. The tendons are placed in the concrete and then stressed using hydraulic jacks. The tendons are not bonded to the concrete, which allows for movement of the concrete and helps to prevent cracking.
The Lee-McCall system is a method of prestressing concrete using bonded tendons. The tendons are placed in the concrete and then stressed using hydraulic jacks. The tendons are bonded to the concrete, which helps to transfer the stress from the tendons to the concrete.
The Freyssinet system is the oldest and most widely used method of prestressing concrete. It is a reliable and effective method that has been used to construct a wide variety of structures, including bridges, buildings, and dams.