In Sycon, water enters through ostia into a central cavity called

The correct option is A spongoceol
Sycon belongs to the phylum Porifera. They have a perforated body with several openings called ostia. Water enters through ostia into the cavity present at the body's center, known as spongocoel in their water transport system. Osculum is an opening through which the water is removed from the body of a poriferous organism.

The central cavity found in the body of coelenterates is known as the coelenteron. Hypostome is the only external opening present in them.