Which group of marine reptiles had long necks and four flippers?

The group of marine reptiles associated with long necks and four flippers is plesiosaurs. They were aquatic reptiles with broad, flattened bodies, short tails, and four powerful limbs modified into flippers.

The name “plesiosaurs” is often used broadly for members of Plesiosauria, a diverse group that lived from the Triassic into the Cretaceous. Their unusual four-flipper body plan allowed them to move through the water using coordinated strokes, somewhat like flying underwater.

Not every plesiosaur had a long neck. Long-necked forms belonged mainly to the plesiosauromorph type and generally had small heads suited to catching fish and other relatively small prey. Their relatives, the pliosaurs, had short necks and large heads; some were formidable apex predators.

Ichthyosaurs were also marine reptiles, but they evolved a streamlined, fish-like body and a tail-powered swimming style. Mosasaurs had elongated bodies and paddle-like limbs, but they were not the classic four-flipper, long-necked reptiles described in this question.