"Sa tête est ronde."
Translation:His head is round.
French possessive adjectives must match the gender of the object they possess, not the gender of the owner.
La tête is a singular feminine noun, therefore the possessive adjective is sa.
The sa in this sentence...
Sa tête est ronde.
...could mean his or her. Indeed, both are marked correct by this exercise. If the gender of the owner is pertinent, then it must be specified with additional context.
Wrong, "son, sa, ses" can each mean "his" and they can each mean "her" , you won't know if it is "his" or "her" so either will be accepted as correct. Instead the change in "son, sa, ses" corresponds to the gender and number of the item that is possessed. "tête" is feminine and so it takes "la" for its definite article and it takes "sa" for the third person singular possessive adjective. "Sa fille et son garçon sont ses enfants." can mean "His daughter and his son are his children." but it can also mean "Her daughter and her son are her children." https://www.duolingo.com/skill/fr/Possessives-1/tips-and-notes
They sound the same, but in front of a noun that must be “sa”, the possessive adjective, and not the pronoun “ça” which would replace the noun. In fact the pronoun that would be used here would have been “ce” as in “c’est ronde.” which is not this sentence. https://dictionary.reverso.net/french-english/%c3%87a