Which of the following is Mohr's Salt?

The correct answer is option 1. Ammonium iron sulphate.

Ammonium iron(II) sulfate, often known as Mohr's Salt, is an inorganic substance with the chemical formula FeSO₄(NH₄)₂SO₄.6H₂O. Two main cations, ammonium (denoted by NH₄⁺) and ferrous (denoted by Fe₂⁺), are known to be present in this molecule. As a result, Mohr's salt might be regarded as a double salt composed of ferrous and ammonium sulfates. Mohr's salt, which quickly crystallizes and produces crystals that are remarkably resistant to oxidation in the presence of air, is an essential laboratory reagent.