Solution:
concept - When a solid substance directly changes its state from solid to gaseous on heating without going through the liquid phase, then this process is known as sublimation.The substance that can undergo sublimation is called sublimate. Examples- naphthalene, camphor, ammonium chloride ( ), benzoic acid, and iodine.
Thus, in a mixture of salt (sodium chloride) and ammonium chloride, ammonium chloride is a sublimate that sublimes (volatilizes) after some heating leaving behind the common salt (sodium chloride).
Therefore, filtration, sedimentation and Sieving are not useful for separating ammonium chloride and common salt.
Hence, the correct answer is 4.
