Use this subject pronoun when talking to a friend.

O Τύ
O Ella
O Yo
O Ellos

O Tú

The correct subject pronoun to use when talking to a friend is "Tú."

The correct subject pronoun to use when talking to a friend in Spanish is "tú."

To determine the correct subject pronoun to use, it's important to understand the different forms of subject pronouns in the Spanish language:

- "Yo" means "I" in English and is used when referring to oneself.
- "Tú" means "you" in English and is used when talking to someone familiar, such as a friend or family member.
- "Él" means "he" in English and is used when referring to a male.
- "Ella" means "she" in English and is used when referring to a female.
- "Ellos" means "they" in English and is used when referring to a group of males or a mixed-gender group.

So, in this case, the correct answer is "O Tú"