Which air stage is primarily responsible for turbine cooling?

The correct choice related to the stage primarily responsible for turbine cooling is the 13th stage air. In gas turbine engines, cooling is critical to maintaining the operational integrity and efficiency of the turbine components, which are subjected to extremely high temperatures during operation.

The 13th stage typically refers to a point in the axial compressor where the air is at a suitable temperature and pressure to be used effectively for cooling the turbine blades and other parts. It can provide a reliable flow of air that is redirected to cooling passages within the turbine blades, thus ensuring that they do not exceed their temperature limits.

The other stages mentioned offer air that may not be at the right temperature or pressure needed for effective cooling. Thus, the 13th stage plays a vital role not just in maintaining performance, but also in prolonging the life of the turbine by preventing overheating and material degradation.