Which of the following compounds of boron is the hardest?

Which base among the following reacts with diborane to cleave it unsymmetrically?

Excess $NaOH$ can dissolve

The total number of acidic oxides from the following list is: $NO$,$N_2O$,$B_2O_3$,$N_2O_5$,$CO$,$SO_3$,$P_4O_{10}$

The element that combines with oxygen to give an amphoteric oxide is

$A$ Lewis acid '$X$' reacts with $LiAlH_4$ in ether medium to give a highly toxic gas,'$Y$'. '$Y$' when heated with $NH_3$ gives a compound known as inorganic benzene. '$Y$' burns in oxygen and gives $H_2O$ and '$Z$'. '$Z$' is