What will happen if a crystal of ammonium nitrate is added to a supersaturated ammonium nitrate solution? Will it dissolve in water

It will not dissolve. More crystals will separate from the solution.

Step-by-step explanation:

If you add a crystal of ammonium nitrate to a supersaturated solution of the solute, it will not dissolve.

Instead, it will act as a "seed" on which more ammonium nitrate crystals will form.

They will separate from the solution as long, colourless, needle-like crystals until the solution is no longer supersaturated.