Neutralization reaction between ammonia and nitric acid
Your answer is very close to the answer given, except for the following two tidbits (the first being more significant).


An acid-base reaction is not the exchange of a hydrogen atom $\ce{H}$. It is the exchange of a hydrogen ion (or proton) $\ce{H+}$. Thus your answer should be: $$\ce{NH3(aq) +HNO3(aq) -&gt; NH4+(aq) + NO3-(aq)}$$
The given answer combines the two ions produced into a single compound. $$\ce{NH4+(aq) + NO3-(aq) -&gt;NH4NO3(aq)}$$


The result of #2 would be reasonable if you had not been told that the reaction was occurring in aqueous solution. In aqueous solution, the products should have been those of #1, since ammonium nitrate is freely soluble in water.