Final answer:
The true statement about lymph nodes is that the occipital lymph nodes are located at the back of the skull. Other options have inaccuracies regarding the placement and characteristics of lymph nodes.
Step-by-step explanation:
In regards to lymph nodes, the true statement is that the occipital lymph nodes exist at the back of the skull. The supraclavicular lymph nodes are actually located at the base of the neck above the clavicles, not extending up the front of the neck. The anterior cervical chain typically falls along the path of the sternocleidomastoid muscle, which is not just under the mandible but also extends lower towards the clavicle. Hard, fixed lymph nodes are not typically a normal finding and can indicate certain medical conditions. Lastly, the posterior cervical chain is indeed located along the back of the neck, but it runs vertically along the side of the neck rather than at the absolute base.