Final answer:
The correct option is B. The lymph nodes under the mandible or lower jaw are termed submandibular lymph nodes and are a part of the immune system's lymphatic network.
Step-by-step explanation:
The lymph nodes located under the mandible or lower jaw are the submandibular lymph nodes. These nodes are part of the lymphatic system, which plays a key role in the immune response and the drainage of lymph fluid from tissues.
The submandibular glands, which also play a role in saliva production, are located beneath the lower jaw as well, but should not be confused with the lymph nodes.