Halley's comet appears once in about

Answer: (c) 76 years

Halley’s comet returns about every 76 years (last 1986, next 2061).

The idea

A periodic comet returns because gravity keeps it in an orbit around the Sun. Its elongated orbit brings it into the inner solar system only at intervals. Heating near the Sun releases gas and dust, making the comet much easier to see.

Comet travels on an elongated orbit → Approaches the Sun → Surface ice releases gas → Gas and dust form a coma and tails → Comet moves away and fades → Returns on a later orbit

Also know

Halley was last at perihelion in 1986 and is expected to return in 2061; its period is roughly 75–76 years rather than an exact interval. The ion tail points away from the Sun under the solar wind, while the dust tail is curved. Halley’s debris produces the Eta Aquariid and Orionid meteor showers.