Identify the type of mixture formed when nh4no3 is completely dissolved in water

The dissolution of NH4NO3 in water gives a mixture that is homogenous.

They are mixtures in which the dissolved components are uniformly distributed in the entire solvent.

NH4NO3 is soluble in water. It completely dissolves in water and dissociates into its component ions according to the following equation:

NH₄NO₃(s) --->  NH₄⁺ (aq) + NO₃⁻ (q)

The component ions become uniformly distributed in the water, resulting in a homogenous mixture.

More on homogenous mixtures can be found here;