What is the correct order comparing the acid strengths of $HF$,$HCl$,$HBr$,and $HI$?

Arrange the following acids in the decreasing order of their acidic strength: $Cl(OH) \ (I)$,$Br(OH) \ (II)$,$I(OH) \ (III)$.

An atom with atomic number $20$ is most likely to combine chemically with the atom whose atomic number is

Red hot carbon will remove oxygen from the oxides $XO$ and $YO$ but not from $ZO$. $Y$ will remove oxygen from $XO$. Use this evidence to deduce the order of activity of the three metals $X, Y$ and $Z$ putting the most active first.

Which of the following compounds is most acidic?

Which of the following oxides is the most acidic?