Which adjective would best describe "brash" behavior?

The adjective "brash" refers to behavior that is bold, aggressive, and often lacking in sensitivity or consideration for others. It implies a kind of impulsiveness that does not think about the consequences of one’s actions, which can lead to disrespectful behavior. Therefore, describing "brash" behavior as impulsive and disrespectful captures its essence perfectly.

The other choices contrast sharply with this meaning. Behaviors labeled as reserved and deliberate, thoughtful and considerate, or calm and collected suggest a sense of control, thoughtfulness, and respect for others, which is fundamentally opposed to the impulsive and often abrasive nature signified by "brash." Thus, "impulsive and disrespectful" is the most fitting description of such behavior.