NH3 and H2O form NH4OH byelectrovalent bondcovalent bondcoordinate bondnone of these.

NH3 (Ammonia) and H2O (Water) form NH4OH (Ammonium hydroxide) through a coordinate bond.

Here's the step-by-step explanation:

1. A coordinate bond (also known as a dative covalent bond) is a kind of 2-center, 2-electron covalent bond in which the two electrons derive from the same atom.

2. In the case of NH3 and H2O forming NH4OH, the lone pair of electrons on the nitrogen atom in ammonia forms a coordinate bond with a hydrogen atom in water.

3. This results in the formation of the ammonium ion (NH4+) and a hydroxide ion (OH-).

4. These ions then combine to form ammonium hydroxide (NH4OH).

So, the correct answer is "coordinate bond".