Which of the following fuels has the highest calorific value?

$(a)$ $A$ substance $X$,an oxide of a metal,is used extensively in the cement industry. This element is found in our bones also. On treatment with water,it forms a solution which turns red litmus blue. Identify $X$ and also write the chemical reaction involved.
$(b)$ Choose a metal from the following metals which reacts only with hot water: Sodium,magnesium,iron. Mention the products formed during the reaction.

Which metal ore can be concentrated by the magnetic separation method?

Which of the following is not a use of sulfur?

Why does a magnesium ribbon start floating when it is placed in hot water?

Which process is used to obtain metal from metal oxide?