Ammonia gas dissolves in water to form \[N{H_4}OH\]. In this reaction water acts as:
A.A conjugate base
B.A non-polar solvent
C.An acid
D.A base

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Ammonia and water combine to form \[N{H_4}OH\]. In this reaction, the role of ammonia and water can be explained with the help of the Bronsted-Lowry acid-base theory.

Complete answer:
Bronsted–Lowry theory was proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry which deals with the theory of acids and bases. The theory states that a compound that can transfer a proton to any other compound acts as an acid, and a compound that can accept a proton acts as a base in a reaction. A proton is a positively charged hydrogen atom denoted by \[{H^ + }\].
The reaction between ammonia gas and water molecule can be expressed as:
$N{H_3}(g) + {H_2}O(l)\xrightarrow{{}}N{H_4}^ + (l) + O{H^ - }(l)$
base acid conjugate acid conjugate base
Ammonia and water molecules react with each other to give ammonium hydroxide. It is an acid-base reaction. According to Bronsted–Lowry concept ammonia accepts a proton to form $N{H_4}^ + $ ion so it acts as a base and the ammonium ion is the conjugate acid. The water molecule donates a proton to ammonia to generate \[O{H^ - }\] ion. Hence water acts as an acid and the \[O{H^ - }\]is the corresponding conjugate base.
Also, water is a very polar molecule and it normally dissolves all polar compounds by making H-bonds. Thus water is not a non-polar solvent.

So, the correct answer is C, in this reaction water acts as an acid.

Note: The proton donating power of a compound makes it an acid and a proton accepting power makes it a good base. In other words an acid viz.\[HCl\] is an acid because it donates a proton that is accepted by a base that accepts the proton.