Which of following weakest base
A)NaoH
B)NH4OH
C) KOH
D)Ca(OH)2

Don't involve pH here , this way it's easy :

Look bases dissociate to give OH ions in water right .. Now who will be a weak base ?? The one which will not give OH ion easily or slowly.

This means compound must have a strong LATTICE. Look at charges Na, NH4 and K have +1 charge Ca have +2 charge after dissociation . Greater charge Stronger lattice , so Na and K are out of race . We are now left with Ca .  So Ca(OH)2 have strongest lattice - dissociates least - making it aweak base .