‘Oust’ means to expel or remove from a place or position occupied. ‘Ousted’ is a complete verb. ‘In’ is used for unspecific times during a day, month, season, year; to indicate a location or place; to express while doing something or to indicate a belief, opinion, interest, or feeling. So, the use of ‘in’ over here is inappropriate. When ‘power’ is concerned then ‘oust’ is followed by ‘from’. Thus, option (b) is correct.