The correct answer is option 1, Prawn. An open circulatory system is a system in which the fluid called hemolymph directly bathes the organs and tissues. In animals with an open circulatory system, the blood does not flow in closed vessels but rather freely circulates in open spaces called sinuses or hemocoel.
Option 2, Earthworm, is incorrect because earthworms have a closed circulatory system where blood flows in vessels.
Option 3, Toad, is incorrect because amphibians like toads have a closed circulatory system with blood flowing in vessels.
Option 4, Man, is incorrect because humans have a closed circulatory system as well, with blood flowing in vessels.
Therefore, only option 1, Prawn, has an open circulatory system.