"우리의 개는 일곱 살이에요."
Translation:Our dog is seven years old.
Safibta is right. It literally means our. But antlee is right too. Korean use 우리 to say "my" too, especially for family members and to show respect. For example 우리 아빠 (my Dad) or 우리 나라 (my country). So both is correct but I think, here duolingo wants to show the literal translation.
Specifically, the ambiguity comes in whether the pronoun includes or excludes the listener. So while "우리" and "저의" may both be used to mean "my", the connotation is different.
"저의 나라" can mean "my country, but not your country"
"우리 나라" can mean "my country, whether or not it is yours as well"