The correct answer is: Sofia is the capital of Bulgaria.
The article "the" is used before a noun to indicate that it is the only one of its kind or that it is being referred to specifically. In this case, Sofia is the only capital of Bulgaria, so the article "the" is used.
The article "a" is used before a noun to indicate that it is one of a number of similar things or that it is being referred to generally. In this case, Sofia is one of many capital cities in the world, so the article "a" would not be used.
The article "no article" is used before a noun when it is not being referred to specifically or when it is the first mention of the noun. In this case, Sofia is being referred to specifically, so the article "no article" would not be used.
