The possessive his/her/its in french is >sa< behind singular feminine nouns >son< behind singular masculine nouns or >ses< behind plural masculine or feminine nouns.
The noun in question was "chaussure", a singular feminine noun. Therefore, the appropriate possessive will be "sa".
The noun can then not be "chaussures" which is a plural noun because the possessive behind it is a singular feminine one.
Hope this helps