Elastic Shortening of Concrete
Elastic shortening of concrete refers to the contraction or compression of concrete when subjected to an external load or force. This deformation occurs due to the elastic properties of the material. In pre-tensioning, the concrete is initially cast around the pre-stressing tendons. As the tendons are tensioned, they impart compressive forces onto the concrete. This compression causes elastic shortening of the concrete, resulting in a decrease in length. However, in post-tensioning, the tendons are tensioned after the concrete has hardened, so the concrete has already undergone most of its elastic shortening. Therefore, elastic shortening of concrete occurs only in pre-tensioning and not in post-tensioning.

Shrinkage of Concrete
Shrinkage of concrete refers to the reduction in volume or dimensions of the concrete due to the loss of moisture. When concrete dries, the water within it evaporates, leading to shrinkage. This shrinkage can cause cracks or deformations in the concrete. Both pre-tensioning and post-tensioning are susceptible to shrinkage, as the concrete used in both methods contains water that can evaporate. Therefore, shrinkage of concrete occurs in both pre-tensioning and post-tensioning.

Creep of Concrete
Creep of concrete refers to the gradual deformation or elongation of concrete under a constant load over time. This phenomenon is a result of the viscoelastic properties of concrete. Creep occurs in both pre-tensioning and post-tensioning, as the applied load on the tendons can cause the concrete to deform over time.

Loss due to Friction
Loss due to friction occurs in both pre-tensioning and post-tensioning. It refers to the reduction in the prestressing force due to the frictional resistance between the tendons and the surrounding concrete or ducts. This friction can result in a loss of prestress, affecting the overall performance of the structure.

Conclusion
Among the given options, only the elastic shortening of concrete occurs exclusively in pre-tensioning and not in post-tensioning. Shrinkage of concrete, creep of concrete, and loss due to friction can occur in both pre-tensioning and post-tensioning. It is important to consider these factors when designing and implementing pre-stressed concrete structures to ensure the long-term performance and durability of the structure.