Madagascar est une ’île .

I don't understand - how does that answer the question?

Do you mean it should be "La Madagascar"? 

According to https://www.thoughtco.com/say-countries-of-world-in-french-4080349:

"Some of the countries do not have a definite article because they are islands and articles are normally not used with islands."

Just something to remember, I suppose. I don't remember seeing that on Kwiziq.

Egalement avec Haïti, St. Martin, Vanuatu, St. Pierre & Miquelon... la plupart des îles n'ont pas d'article.