Which one of these is formal? adi�s, señor. hasta luego, paco. chao

Formal: adiós, señor.

Explanation:

Formal language is that which is used in formal events or situations that require seriousness. Usually, speakers who use formal language in a conversation do not know each other very well, respect their hierarchy or find themselves in a framework of extreme cordiality. Some of the situations in which the use of formal language can be seen are the conversations between a doctor and his patient, between two leaders from different countries, between two strangers, between a teacher and his student, etc.