Dolphins are highly intelligent marine mammals that belong to the family Delphinidae. They are known for their playful behavior, social interactions, and remarkable communication skills. Dolphins possess several distinctive features that set them apart from other animals. Let's examine each option to determine which one is not a feature of dolphins.

a) Giving birth to young ones: One of the key features of dolphins is their ability to give birth to live young ones. Like other mammals, dolphins are viviparous, meaning they undergo internal fertilization and carry the developing offspring inside their bodies until they are ready to be born. They give birth to a single calf, which is usually born tail-first to prevent drowning. This feature is unique to mammals, including dolphins.

b) Lay eggs: This option is incorrect. Dolphins do not lay eggs. Egg-laying is characteristic of animals such as reptiles (e.g., turtles, snakes) and birds. Dolphins, being mammals, do not lay eggs.

c) Have fins: Dolphins are well-known for their streamlined bodies and fins, which aid in their swimming and maneuverability. They have a dorsal fin on their backs, pectoral fins on their sides, and a fluke (tail fin) at the end of their bodies. These fins help dolphins to steer, change direction, and propel themselves through the water. Fins are an essential feature of dolphins.

d) None of these: This option is incorrect. Dolphins possess all of the mentioned features, except for laying eggs. Therefore, this option cannot be the correct answer.

In conclusion, the correct answer is option B, which states that dolphins do not lay eggs. Dolphins give birth to live young ones, have fins for swimming, and possess various other adaptations that enable them to thrive in their marine habitats.