An open circulatory system is a type of circulatory system in which the blood is not entirely contained within blood vessels. Instead, the blood is pumped from the heart into the body cavities, where tissues are directly bathed in blood. This is typical of many invertebrates, including arthropods and some mollusks.

Let's analyze the options provided:

Option A: Palaemon, Nereis, Balanoglossus

- Palaemon is a type of prawn (crustacean, arthropod).

- Nereis is a polychaete worm (annelid).

- Balanoglossus is a hemichordate.

Here, Palaemon has an open circulatory system, but the other two do not fit this category.

Option B: Hirudinaria, Bombyx, Salpa

- Hirudinaria, a leech (annelid).

- Bombyx, the silkworm moth (arthropod).

- Salpa, a type of tunicate (urochordata).

Bombyx has an open circulatory system, but the other two do not.

Option C: Anopheles, Limax, Limulus

- Anopheles, a mosquito (arthropod).

- Limax, a type of slug (mollusk).

- Limulus, the horseshoe crab (arthropod).

All three organisms listed in Option C have an open circulatory system.

Option D: Pheretima, Musca, Pila

- Pheretima, an earthworm (annelid).

- Musca, the common housefly (arthropod).

- Pila, an apple snail (mollusk).

Musca and Pila have an open circulatory system, but Pheretima does not.

Based on this analysis, the correct answer is:

Option C: Anopheles, Limax, Limulus