Option 1 -> unscrupulous means lacking moral principles or being dishonest, not related to impulsive behavior.
Option 2 -> obstinate means stubbornly refusing to change one's mind, which describes rigidity rather than impulsiveness.
Option 3 -> impetuous means acting quickly without thought or care, perfectly describing someone who acts on sudden ideas.
Option 4 -> conceited means excessively proud or vain, unrelated to acting without thinking.
Hence, Option 3: impetuous -> The word "impetuous" specifically describes hasty, impulsive behavior done without careful consideration, which directly matches acting on sudden ideas without thinking first -> correct