a) advertisement - advertise, cleverness - clever, correction - correct, decision - decide, difference - differ, entertainment - entertain, entrance - enter, excitement - excite, friendliness - friendly, happiness - happy, illness - ill, imagination - imagine, importance - important, invention - invent, operation - operate, permission - permit, possibility - possible, presentation - present, production - produce, reality - real, sadness - sad.
b) 1. The advertisement was very eye-catching. 2. Her cleverness impressed everyone in the class. 3. The correction of the mistake took a long time. 4. My decision to study abroad changed my life. 5. The difference between the two products was significant.