Correct Option: C

slow (Adjective) : not done quickly
rapid (Adjective) : happening in a short period of time ; done or happening very quickly
happy (Adjective) : cheerful
fall (Noun/Verb) : act of falling; waterfall; a decrease in size, number, rate/level to drop down from a higher level to a lower level
abnormal (Adjective) : different from what is usual or expected, especially in a way that is worrying, harmful or not wanted