What instrument is used to check the set of cement before removal?

The correct choice is an explorer, which is specifically designed to assist in examining the surface of teeth and dental materials. When checking the set of cement before removal, the explorer can be used to gently probe and assess whether the cement has adequately hardened and to identify any areas that may still be soft or unset. This checking process is crucial to ensure the integrity of the dental restoration before further procedures take place.

The explorer's sharp tip allows for precise examination, making it easier to determine the hardness of the cement. The other instruments, such as a scalpel, spoon excavator, or mirror, do not serve the same purpose in assessing the readiness of the cement. A scalpel is typically used for cutting soft tissues, a spoon excavator is mostly for removing decay or debris from cavities, and a mirror is used primarily for visualization of hard-to-see areas in the mouth. Therefore, the explorer stands out as the most appropriate tool for this specific task.