Willy is:

Answer: (c) a tropical cyclone of the north-west Australia

Willy-willy is the local name for tropical cyclones of north-west Australia.

The idea

Common names for rotating winds can be less precise than scientific names. A tropical cyclone is an organised low-pressure storm over warm ocean water. A dust devil is a small whirl generated by strong heating over land. The old exam association of “willy-willy” with Australian cyclones differs from common modern Australian use for dust whirls.

Why the others are wrong

(b) Dust devils in Australian deserts are also called willy-willies — but the key means the tropical cyclone.

The trap

In Australia the word is also used for small dust whirls.

Also know

Hurricane is the regional name used in the Atlantic and north-eastern Pacific; typhoon is used in the north-western Pacific. The Indian Ocean and Australian region use tropical cyclone in formal weather reports. A tornado usually extends from a thunderstorm, whereas a dust devil does not require a thundercloud. These phenomena differ greatly in size and formation.