This fish shows dorsal fin modified into suckers

The correct option is Echeneis.

Step-by-Step Explanation:

1. Understanding the Adaptation:
Some marine organisms develop specialized morphological adaptations to survive in their environment. A prominent example is the modification of the first dorsal fin into an oval, transversely laminated adhesive disc or sucker.

2. Function of the Sucker:
This modified dorsal fin is located on the head region. It allows the fish to attach itself to larger marine animals, such as sharks, turtles, and whales, or even to the hulls of ships. This commensal relationship helps the fish travel long distances without expending much energy and provides protection from predators, as well as access to food scraps left by the host.

3. Identifying the Organism:
The fish commonly known as the remora or suckerfish belongs to the genus Echeneis. In these fish, the dorsal fin is uniquely structured to function as a powerful vacuum cup or sucker.

4. Analyzing the Other Options:
- Neoceratodus is a genus of lungfish and does not possess a dorsal sucker.
- Hippocampus represents sea horses, which have a prehensile tail and lack a dorsal sucker.
- Torpedo is the electric ray, characterized by electric organs modified from muscle tissue, not a dorsal sucker.

Therefore, Echeneis is the correct fish that shows a dorsal fin modified into suckers.