Answer:
both
Step-by-step explanation:
One of the rules to pluralize a noun that ends with "-o" dictate that you have to ad -"es" to the word. Such as in the word "Potato", that in Plural form it becomes the word "Potatoes" with the addition of the "-es".
Some words, though, don't necessarily follow the rule. One of them being Vulcano that can be written as both "Vulcanos" and "Vucanoes". Same goes to "Grotto" which can be both "Grottos" and "Grottoes".