The constellation that helps to locate North Star is

Constellation is a group of stars which forms a recognisable pattern. They also help to identify direction, time month approximately.

Cassiopeia is the constellation which is located in the northern region of sky and therefore can be used to locate North Star. Other constellations like Orion is used to locate Sirius star, and Taurus and Leo are located on the zodiac belt which follows a path from east to west and are therefore is not used to locate the north star.

Therefore, correct option is (a) Cassiopeia.