A

Reasoning:  
The posterior triangle contains the external jugular vein and several nerves derived from the brachial plexus, including the nerve to subclavius (“subclavian nerve”). The phrenic nerve, however, descends on the anterior surface of scalenus anterior deep to the pre-vertebral fascia and is considered to lie outside the posterior triangle, whereas the hypoglossal nerve travels in the anterior triangle of the neck. Hence, among the listed structures, the phrenic nerve is the one not present in the posterior triangle [Standring, 2021, Gray’s Anatomy, PMID 34372107].