Hint: You should have basic knowledge of Mohs scale of mineral hardness and then according to the malleable and ductile nature of metals you can think of the answer. And softness is nothing but the ability of a substance to get easily mould, cut, compress or fold.
Complete Step by Step Answer: The softness of a metal refers to the ability of a metal material to deform easily through the intrusion of a hard object. As asked in question which is the softest metal among platinum, silver, iron and aluminium. To solve this question, we have to define Mohs scale of mineral hardness. The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. You should remember some general data that helps in solving many questions related hardness and softness of metal. Hardness of caesium and rubidium is 0.2-0.3 (according to Mohs hardness). Hardness of lithium, sodium, potassium is 0.5-0.6 and that of talc is 1. Hardness of gold, silver, aluminium, zinc, lanthanum, cerium, jet is in between 2.5-3 and that of platinum is 3.5. Hardness of iron is 4. Hence, we conclude that iron is hardest amongst them. As hardness of aluminium and silver is in between 2.5-3, then we go on detailed analysis of Mohs hardness and conclude that hardness of Aluminium is 2.75 and that of silver is 2.5. Hence from here we conclude that silver is the softest metal among the given options.
Hence, Option B. Silver is the correct answer.
Note:
* Caesium is the softest metal with the Mohs hardness of 0.2.
* Always remember, the hardness of gold, silver, aluminium, zinc, lanthanum, cerium, jet is in between 2.5-3 and that of platinum is 3.5 (according to Mohs hardness).
* Always remember that Silver is softer than Aluminium.
