Select the correct meaning of the idiom “On cloud nine”.

This question requires understanding the meaning of the idiom “On cloud nine.” Idioms have figurative meanings that differ from the literal interpretation of their words.

• To be extremely happy is the correct answer. The idiom 'on cloud nine' signifies a state of intense joy, euphoria, or bliss.
• To be lost in thoughts (A) describes being preoccupied or distracted, not necessarily happy.
• To travel in space (B) is a literal interpretation of 'cloud nine', which is incorrect for an idiom.
• To be far from home (D) describes a physical location, which is unrelated to the emotional state conveyed by the idiom.

Hence, 'to be extremely happy' accurately defines the idiom 'on cloud nine'.