Final answer:
The most commonly used implant to support a mandibular complete denture is the Endosseous implant, which is inserted into the bone and acts as an anchor for the denture.
Step-by-step explanation:
The implant that is most commonly used to support a mandibular complete denture is the Endosseous implant. This type of implant is inserted into the bone and acts as an anchor for the denture. It is made of biocompatible materials such as titanium or ceramic, and provides stability and support for the denture.