Final answer:
The longus colli muscle is an anatomical landmark for locating the cervical sympathetic trunk and its associated ganglia, which are part of the sympathetic nervous system.
Step-by-step explanation:
The longus colli muscle is situated on the anterior side of the vertebral column, and its anatomy is crucial in locating several structures in neck surgery. Among the structures associated with the vicinity of the longus colli muscle, the most relevant to your question is the cervical sympathetic trunk.
The cervical sympathetic trunk and its ganglia (collateral or prevertebral ganglia) are part of the sympathetic nervous system, which runs alongside the vertebral column. It significantly contributes to the autonomic functions in the body, controlling organs responsibly with various homeostatic mechanisms.
Overall, the longus colli muscle serves as a guide for locating the cervical sympathetic trunk and associated ganglia, hence option (a) is the correct answer to your question.