rational B1

/ˈɹæʃ(ə)nəl/ Based on clear, sensible thinking rather than emotion.

rationale C2+

/ˌræʃəˈnæl/ the set of reasons or logical basis underlying a course of action

rationalization C2+

/ˌræʃənələˈzeɪʃən/ the act of justifying behavior with plausible but specious reasons