(s) about s-block compounds.

A is correct: the small, strongly polarising $\mathrm{Li^+}$ stabilises the oxide and the nitrate decomposes as $\mathrm{4LiNO_3\to2Li_2O+4NO_2+O_2}$, while the sodium and potassium nitrates stop at the nitrite. B is incorrect: the superoxide ion $\mathrm{O_2^-}$ has $13$ valence electrons, three of them in the $\pi^*$ level, so one electron is unpaired; it is paramagnetic with bond order $1.5$. C is incorrect: in the chain of solid $\mathrm{BeCl_2}$ each Be is bonded to four bridging chlorine atoms (3c-4e type bonding), tetrahedral and $sp^3$ hybridised; the planar $sp^2$ picture applies to the gas-phase monomer. D is incorrect: the peroxide ion $\mathrm{O_2^{2-}}$ has bond order $1$ and all electrons paired in $\pi^*$, so it is diamagnetic. Only A is correct.