Strictly speaking, Formosa was only ever the name (in English) of an island; it was never a country. Parts of the island were under Dutch and Spanish rule during the mid-17th century, and these colonies were known as Dutch Formosa and Spanish Formosa. But there has never been a country called Formosa.
Today, the official name of Taiwan is The Republic of China (as opposed to the People's Republic of China). This should have been allowed as an alternative answer.
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