The skeleton entirely made of cartilage is in (a) Sharks.
Explanation: Sharks, along with other cartilaginous fish like rays, have skeletons composed entirely of cartilage, unlike bony fish like tuna and rohu which have bone skeletons.
Why other options are incorrect:
(b) Tuna: Tuna is a bony fish, meaning its skeleton is made of bone, not cartilage.
(c) Rohu: Rohu is also a bony fish, with a bone skeleton.