Which instrument is best for determining if the interior diameters of two pipes are identical?

A caliper is the most suitable instrument for determining if the interior diameters of two pipes are identical because it is designed to measure dimensions with high precision. Calipers can measure inside diameters effectively using their inside jaws, making them ideal for comparing two pipes directly. The ability to position the caliper's arms inside each pipe allows for a direct comparison, ensuring that even slight variations in diameter can be easily detected.

In contrast, while a micrometer offers precision, it is typically used for measuring smaller objects or thicknesses rather than the interior dimensions of pipes. A gauge may refer to various measuring devices, but it generally lacks the precision required specifically for internal diameter measurements. A measuring tape is useful for larger dimensions but does not provide the accuracy necessary for comparing smaller internal diameters between pipes. Therefore, the caliper stands out as the most effective tool for this specific measurement task.