The correct option is
A Hollow
Hydra has a bassel-shaped body topped by a mouth which is
surrounded by Tentacles. They contains difficult type of stinging
cells on their tentacles called nematocysts. The body of a Hydra
is a hollow cylinder with a sold base and an opening at the
mouth end. The tentacles which mode of nematocyst we also
hallow and elongated which we used to catch prey.
Hence the tentacles of Hydra are Hallow.