A person whose "head" is in the "clouds" is -- ?

The phrase "head in the clouds" is an idiomatic expression that typically refers to someone who is lost in their thoughts, often daydreaming or imagining scenarios instead of focusing on reality. This state of mind is generally characterized by a lack of attention to practical matters or the immediate environment, indicating a tendency to fantasize or ponder abstract ideas without grounding in everyday situations.

Among the provided options, "B) a day dreamer" is the most fitting. A daydreamer is someone who often envisions possibilities that may not correspond with their current circumstances, akin to having one's "head in the clouds." This contrasts with the other choices: "A) proud" does not convey a specific state of mind related to imagination or distraction; "C) an aviator" suggests a literal interpretation that doesn't align with the metaphor; and "D) useless" carries a negative connotation that does not reflect the generally innocent or whimsical nature of daydreaming.

Thus, "a day dreamer" encapsulates the essence of someone who appears disconnected from the present due to their imaginative thoughts, making it the correct answer.