What is the colour of product when ammonia reacts with a solution of $Cu^{2+}$?

Deep blue

The correct answer is Option (3) → Deep blue

When aqueous ammonia is added to a solution containing $Cu^{2+}$ ions, a two-step reaction occurs:

1. Initial Addition: A small amount of ammonia acts as a base, reacting with the hydrated copper ions to form a pale blue precipitate of copper(II) hydroxide, $Cu(OH)_2$.
2. Excess Ammonia: When excess ammonia is added, the precipitate dissolves to form a soluble complex ion called tetraamminecopper(II). This specific coordination complex is responsible for the characteristic deep blue (or "royal blue") color of the solution.

The chemical equation for the formation of the deep blue complex is:

$[Cu(H_2O)_6]^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4(H_2O)_2]^{2+} + 4H_2O$