Final answer:
The term "Ud." is the abbreviation for "Usted" in Spanish, which is the formal form of address. It is appropriately used with strangers to show respect, while informal situations with friends, family, and pets use "tú". The correct option to use "Ud." is with D) A stranger.
Step-by-step explanation:
In Spanish, different forms of address are used depending on the level of formality of the situation and the relationship to the person you are speaking to. The formal "Usted" (often abbreviated as "Ud.") is used to show respect towards someone, which is typically required in professional settings or when addressing strangers. In contrast, the informal "tú" is used with friends, family members, and pets, due to the familiar and close nature of these relationships.
Given the options provided:
A friend - use the informal "tú"
Your sister - use the informal "tú"
Your dog - use the informal "tú"
A stranger - use the formal "Ud."
Therefore, the correct option to use "Ud." is with D) A stranger.