Which was one of the first animals to develop a true body cavity?

cnidarians

mollusks

sponges

flatworms

Mollusks is one of the first animals to develop a true body cavity.

Explanation:

True body cavity is also referred to as "Coelom". It is nothing but a hallow, fluid filled cavity that is found in almost all living organism which mainly functions as a protective cushion for all the internal organ of that organism. This body cavity also allows the organs of that organism to grow and move independently. In some animals or organisms such as worms, it also act as a skeleton. From the above provided options Mollusks is the animal that is seen to have developed a true body cavity. Mollusks is mostly of aquatic nature and it is named after a Latin word "Molluscus" which means soft. This soft body of them is covered and protected by the body cavity called Coelom.

Hence we can conclude Mollusks as right answer.