Which is the lightest gas?

Fire extinguishers contain $H_2SO_4$ and which one of the following?

Which of the following is commonly used in a laboratory desiccator?

The most abundant element in the universe is

In $para-hydrogen$,what is the nature of the electron spin?

Dihydrogen can be prepared by which of the following reactions?
$(I)$ Reaction of granulated $Zn$ with dil. $HCl$
$(II)$ Reaction of $Zn$ with aq. $NaOH$
$(III)$ By heating calcium hydrogen carbonate