The alkali metals are physically soft, silvery white and light metals. Due to the large atomic size, these elements have low density which increases down the group from Li to Cs. Caesium has the largest atomic size out of the given examples, and had weak metallic bonding ability due to the presence of a single valence electron. Hence it has the lowest melting point of 302°K or 28°C, hence it will melt if the room temperature rises to 30°C. The correct answer is (iv).