It's E. ClO4- is a salt of the strong acid, therefore, the weakest base. HClO4 is a strong acid, not a base. NH3 is a strong base...all the others got charges...
Well you have to memorize the 6 strong acids and Seven strong bases or so. Once you know that then the conjugate base of a strong acid is ver very weak. Therefore ClO4- is the conjugate base of HClO4- the strongest acid there is so it is the weakest base by far. HClO4 isnt a base its an acid and NH3 is a medium strength base. However Oh is a strong base and NH2- is one of the sytrongest bases there is. The only correct answer here is E-ClO4-.
Approach it this way. Right away you see HClO4 as clearly the strongest acid out of the choices. So, HClO4 is the stongest acid, then ClO4- is going to be the weakest base.