Learn Swedish: Vilka svar är rätt? - Which answers are right?

Vilka is the plural form of "which". We use it here because we are asking about multiple answers. If we were asking about just one answer, we would use vilket because svar is an ett-word (vilket svar).

How can you tell svar means "answers" (plural) when the singular word for "answer" is also svar?

Svar is an ett-word (neuter noun) that ends in a consonant. In Swedish, these types of words don't change their spelling in the plural. You have to look at the context clues to know it is plural. Here, the plural word vilka tells us that svar must be plural too.

Since we are talking about plural answers, why isn't rätt spelled with an "-a" at the end (rätta)?

Normally, adjectives get an -a added to the end when describing plural nouns. However, rätt (right) and fel (wrong) are common exceptions. When they come after the verb är (are), they usually don't change form, even for plural words.