A water sample from a hot thermal vent contained a single-celled organism that had a cell wall but lacked a nucleus. What is its most likely classification?

Archaea

Explanation:

Archaea are prokaryotic single cell microorganism.

They are usually found in extreme environment like hot thermal vent and others. They share characteristics with bacteria and eukaryote. Archaea as a prokaryotic organism they lack true nucleus and organelles but unlike bacteria they are unaffected by antibiotics and contain different cell wall components. Unlike bacteria and eukaryotes, their membranes contain branching lipids.