Which one of the following combines with $Fe(II)$ ions to form a brown complex?

The most acidic among the following compounds is

Which of the following is a tetrabasic acid?

Which oxide of nitrogen is obtained on heating ammonium nitrate at $250 \, ^oC$?

Account for the following observations:
$(A)$ $AlCl_3$ is a Lewis acid.
$(B)$ Though fluorine is more electronegative than chlorine yet $BF_3$ is a weaker Lewis acid than $BCl_3$.
$(C)$ $PbO_2$ is a stronger oxidizing agent than $SnO_2$.
$(D)$ The $+1$ oxidation state of thallium is more stable than its $+3$ state.

Which hydride among the following is the strongest reducing agent?