The following five compounds, one mole each, are completely hydrolysed by an excess of dilute aqueous H2SO4 in a closed set-up that lets any gas…

Write each hydrolysis and then ask whether the gas survives the acid.

$\mathrm{CaC_2}+2\mathrm{H_2O}\to\mathrm{Ca(OH)_2}+\mathrm{C_2H_2}$: 1 mol of ethyne, unaffected by dilute acid.
$\mathrm{Al_4C_3}+12\mathrm{H_2O}\to4\mathrm{Al(OH)_3}+3\mathrm{CH_4}$: 3 mol methane, collected.
$\mathrm{CaH_2}+2\mathrm{H_2O}\to\mathrm{Ca(OH)_2}+2\mathrm{H_2}$: 2 mol hydrogen, collected.
$\mathrm{Mg_3N_2}$ gives 2 mol $\mathrm{NH_3}$ and $\mathrm{Li_3N}$ gives 1 mol $\mathrm{NH_3}$, but ammonia is a base and is completely retained by excess $\mathrm{H_2SO_4}$ as $\mathrm{(NH_4)_2SO_4}$, so no gas from these two is collected.

Total gas collected: $1+3+2=6$ mol. Counting the ammonia as well would give 9, the typical error. The answer is $6$.