Here's why:
* Plural: The plural of "contralto" is "contraltos".
* Possessive: To make a noun possessive, we add an apostrophe (').
* Possessive Plural: For plural nouns ending in "s", we add just an apostrophe (') after the "s" to make it possessive.
Therefore, the possessive plural of "contralto" is "contraltos'".