Epithelial cells are stitched together by junctions with distinct jobs, and the question targets the one that seals. The zonula occludens, or tight junction, lies at the very apex of the lateral cell border where the outer leaflets of neighbouring membranes are zipped shut by transmembrane proteins claudin and occludin. This fusion closes the paracellular route, enforces selective permeability, and maintains the apico-basal polarity that lets epithelia act as true barriers.
The other choices serve adhesion or communication rather than sealing. Fascia adherens belongs to the intercalated disc and anchors actin filaments in cardiac muscle; the desmosome, or macula adherens, is a rivet-like spot that ties intermediate filaments together for mechanical strength; and the gap junction is a channel array of connexons that allows ions and small molecules to flow between cells. None of these occludes the intercellular cleft, so the unique occluding junction is the zonula occludens.
\[\boxed{\text{Zonula occludens (tight junction)}}\]