Answer:C: the number of the noun
Step-by-step explanation:
When we use possessive adjectives like tu, tus, mis etc we are matching them with the number of the noun, plural or not, example,
"Tus calcetines"
TUS is a plural possesive adjective and calcetines is the plural noun of calcetin
"TU calcetin"
TU is singular, and calcetin is a singular noun