$PbO_2$ is

Given below are two statements :
Statement $I$: In group $13$, the stability of $+1$ oxidation state increases down the group.
Statement $II$: The atomic size of gallium is greater than that of aluminium.
In the light of the above statements, choose the most appropriate answer from the options given below:

Concentrated $HNO_3$ turns brown on standing due to the formation of:

Which of the following is an acidic oxide?

How is nitrogen prepared in the laboratory? Write the chemical equations of the reactions involved.

Given below are two statements:
Statement $I$: Like nitrogen that can form ammonia,arsenic can form arsine.
Statement $II$: Antimony cannot form antimony pentoxide.
In the light of the above statements,choose the most appropriate answer from the options given below: