Which of the following people would you most likely address as "tu"?

your sibling
your mother's friend
your doctor

I would most likely address my sibling as "tu".

You would most likely address your sibling as "tu."

To determine which of the following people you would most likely address as "tu," you need to consider the relationship and level of familiarity with each person.

1. Your sibling: In most cases, you would address your sibling as "tu." They are usually close in age and have a more informal and familiar relationship, making the use of "tu" appropriate.

2. Your mother's friend: This would depend on the cultural context and the level of familiarity between you and your mother's friend. In many cultures, it is common to address your parents' friends using a more formal and respectful form of address, such as "usted" in Spanish. However, if you have a close relationship or the friend requests to be addressed informally, you may use "tu."

3. Your doctor: It is generally considered more appropriate to address your doctor using a more formal and respectful form of address, such as "usted." This is because doctors are professionals and maintaining a certain level of formality in the doctor-patient relationship is customary.

In summary, you would most likely address your sibling as "tu." The decision whether to address your mother's friend or your doctor as "tu" or "usted" depends on the specific circumstances and cultural norms.