Final answer:
The reaction between ammonia and water is neither synthesis, double replacement, single replacement, nor decomposition; it is an acid-base reaction.
Step-by-step explanation:
The reaction between ammonia (NH3) and water (H2O) to form ammonium hydroxide (NH4OH) is an example of an acid-base reaction. Specifically, this is a reaction in which water acts as an acid, donating a proton (H+) to the ammonia, which functions as a base. Thus, the answer to the question is not explicitly listed among the given choices, as it is not a synthesis, double replacement, single replacement, or decomposition reaction. The correct term for this type of reaction is acid-base reaction or sometimes referred to as a hydrolysis reaction.