The correct answer is: D. Loss due to friction
Loss due to friction is the only loss of prestress that occurs only in pretensioning and not in post-tensioning. This is because, in pretensioning, the tendons are stressed before the concrete is poured, and the friction between the tendons and the concrete causes the tendons to elongate slightly. This elongation results in a loss of prestress. In post-tensioning, the tendons are stressed after the concrete has been poured, so there is no friction between the tendons and the concrete, and no loss of prestress due to friction.
Elastic shortening of concrete is the shortening of concrete that occurs when it is stressed. This shortening is caused by the deformation of the concrete fibers. Elastic shortening is a reversible process, meaning that the concrete will return to its original length when the stress is removed.
Shrinkage of concrete is the reduction in volume of concrete that occurs over time. This shrinkage is caused by the loss of water from the concrete. Shrinkage is an irreversible process, meaning that the concrete will not return to its original volume once the water has been lost.
Creep of concrete is the deformation of concrete that occurs over time under sustained load. This deformation is caused by the slow movement of the concrete fibers. Creep is an irreversible process, meaning that the concrete will not return to its original shape once the load has been removed.