What type of gauge is used to measure the clearance between the high pressure and low pressure turbines?

The feeler gauge is the correct choice for measuring the clearance between the high-pressure and low-pressure turbines. This tool consists of a series of thin, flat metal blades of varying thicknesses that can be inserted between two mating surfaces to determine the gap or clearance.

In the context of turbine components, maintaining proper clearance is crucial for efficient operation and preventing damage. The feeler gauge allows for precise measurement of these narrow spaces, ensuring correct alignment and function of the turbines.

While the other options serve important roles in measurement and machining, they are not suitable for this specific application. Digital calipers and micrometers are typically used for measuring larger or more general dimensions, while a taper gauge is designed for measuring the diameters of tapered surfaces, which is not applicable to turbine clearances. Thus, the feeler gauge stands out as the appropriate instrument for this task.