According to the given information, the products formed from the reaction are
KNO3 and Mg(OH)2.
KNO3 is a salt that is soluble in water.
Mg(OH)2 is an hydroxide that is not soluble in water.
It means that a precipitate of Mg(OH)2 will be formed in the reaction, then, the products of the reaction are:
KNO3(aq)+Mg(OH)2(s).
The correct answer is D.