The passengers were very happy ___ the friendly and warm treatment given to them.

The correct preposition to complete the sentence is "about". The phrase "happy about" is an idiomatic expression used to indicate the reason or cause of one's happiness. The passengers were happy because of, or concerning, the treatment they received.

• "to" is often used with infinitives (e.g., happy to help) or to indicate direction.
• "at" can be used for a specific event or situation (e.g., happy at the news), but "about" is more general for a reason.
• "of" is generally not used with "happy" in this context; it's grammatically incorrect here.