To solve this kind of exercise, you have to look the acid and base where the salt come from:

If the cation comes from a strong base, and the anion comes from a strong acid, the salt will be neutral.

If the cation comes from a strong base, and the anion comes from a weak acid, the salt will be basic.

If the cation comes from a weak base and the anion comes from a strong acid, the salt will be acidic.

1. NH4+ is the conjugated acid of NH3 a weak base.

Cl- is the conjugated base of a strong acid.

So NH4Cl will be acidic

2. K+ is the conjugated acid of KOH a strong base.

NO3- is the conjugated base of HNO3 strong acid.

So KNO3 is neutral

3. Na+ is the conjugated acid of NaOH a strong base.

NO2- is the conjugated base of HNO2 a weak acid.

So NaNO2 is basic