Correct choice is (b) veranda
The best I can explain: A veranda or verandah (from Portuguese varandaa) is a roofed, open-air gallery or porch. A veranda is often partly enclosed by a railing and frequently extends across the front and sides of the structure.
Although the form “verandah” is correct and very common, some authorities prefer the version without an h (the Concise Oxford English Dictionary gives the h version as a variant and the Guardian Style Guide says “veranda not verandah”.