The correct answer is option 'A' - They all possess a dorsal heart.

Nereis, scorpion, cockroach, and silverfish are all organisms belonging to different phyla. Let's discuss each of them individually to understand why option 'A' is the correct answer.

1. Nereis: Nereis is a marine polychaete worm belonging to the phylum Annelida. It possesses a well-developed circulatory system with a dorsal blood vessel, also known as a dorsal heart. This dorsal heart pumps the blood throughout the body.

2. Scorpion: Scorpions are arachnids belonging to the phylum Arthropoda. They have an open circulatory system, but they do possess a dorsal vessel or heart. However, it is important to note that the scorpion's heart is not responsible for pumping the blood. Instead, the circulation is maintained by the movement of muscles and the jointed appendages.

3. Cockroach: Cockroaches are insects belonging to the phylum Arthropoda. They also have an open circulatory system, and like scorpions, they possess a dorsal vessel. The dorsal vessel in cockroaches acts as a heart and is responsible for pumping the hemolymph (insect blood) throughout their body.

4. Silverfish: Silverfish are wingless insects belonging to the phylum Arthropoda. Similar to cockroaches, they also possess a dorsal heart. The dorsal vessel in silverfish pumps the hemolymph throughout their body.

In conclusion, all four organisms mentioned in the question - Nereis, scorpion, cockroach, and silverfish - possess a dorsal heart. This means that option 'A' is the correct answer. It is important to note that while they all possess a dorsal heart, they do not belong to the same phylum. Nereis belongs to Annelida, scorpion to Arthropoda, and cockroach and silverfish also belong to Arthropoda.