Which of the following is the coupling that would be used to combine two short shafts to replace a long straight shaft?

A rigid coupling is used to connect two short shafts to effectively replace a long straight shaft because it provides a solid, inflexible connection between the two shafts. This type of coupling is ideal for situations where alignment between the shafts is precise and where no additional flexibility is required. By maintaining a strong connection, rigid couplings ensure that torque is transmitted efficiently without any loss or slippage, which is crucial when high precision and stability are necessary in mechanical systems.

The other types of couplings mentioned may provide flexibility or accommodate angular misalignment, but they are not well-suited for scenarios where a direct, uncompromising connection is needed. Flexible couplings, while useful for absorbing shock or misalignment, introduce some degree of compliance that can reduce torque transmission effectiveness. Universal joints are designed to allow for angular movement, making them inappropriate for situations requiring a straight, rigid connection. Lastly, chain couplings are primarily used in applications that require the transmission of motion over a longer distance with some flex, which would not replace the functionality of a single straight shaft effectively.