The element whose salts cannot be detected by flame test is

Among $KO_2$,$AlO_2^-$,$BaO_2$,and $NO_2^+$,in which species is an unpaired electron present?

Which of the following is the most important biological compound containing $Mg$ found in the living world?

Which of the following is insoluble in water?

What is the correct stability order of $KO_2$, $RbO_2$, and $CsO_2$?

Identify the correct statements from the following :
$I$. Tendency to form halide hydrates gradually increases from $Be$ to $Ba$ down the group.
$II$. Tendency to form stable superoxides increases from $Li$ to $Cs$ down the group.
$III$. Low solubility of $LiF$ is due to its high lattice energy.
$IV$. Solubility of carbonates of group-$2$ elements increases down the group.