You are carefully examining a living organism. Which of the following should convince you that the organism is acoelomate?
a. It has bilateral symmetry.
b. It is triploblastic.
c. It possesses sensory structures at its anterior end.
d. Muscular activity of its digestive system distorts the outer body wall.

Muscular activity of its digestive system distorts the body wall should convince you that the organism is acoelomate.

An animal that does not have body cavity refers to an acoelomate. It is in contrast with the eucoelomates which are the animals that have body cavity. The acoelomate will not have a cavity which is filled with fluid between the digestive tract and the body. Hence there will be a distortion of body wall by the muscular activity of the digestive system.  

Some of the examples of acoelomate  are Cnidaria,Porifera , Platyhelminthes, Placozoa ,Gnathostomulida.Nemertina and Mesozoa. The presence of pseudocoelom in which the body cavity will not be lined by the mesodermal cells are the Pseudocoelomate.