Which type of caliper is specifically used to find shaft centers or locate shoulders?

The correct choice is Vernier caliper because it is specifically designed to measure the distance between two opposite sides of an object, allowing for precise measurements critical in locating shaft centers or shoulders. The Vernier scale enables users to read measurements with greater accuracy than a standard ruler. Its two scales, the main scale and the sliding scale, provide a means for fine measurements, which is especially useful in mechanical applications where precision is crucial.

In contrast, the other instruments serve different purposes: the dial caliper, while also capable of measuring, is often used for its convenience in reading measurements, as it presents data on a dial instead of a Vernier scale. The digital caliper provides electronic readings, enhancing ease of use and speed, but still may not specifically be tailored for locating features like shaft centers. An outside micrometer is specifically designed for measuring external dimensions, but it is not as versatile in finding various point locations on an object compared to the Vernier caliper. Therefore, the Vernier caliper stands out as the best tool for the specific task of locating shaft centers or shoulders.