Greater the Lewis base strength more stable will be the adduct formed with a Lewis acid. The most stable Lewis acid-base adduct among the following is

Greater the tendency to donate lone pair, more stable will be the Lewis acid-base adduct.

Lewis basic strength order is(For the given options),

$\stackrel{{\mathrm{NH}}_3>{\mathrm{PH}}_3>{\mathrm{H}}_2\mathrm{O}>{\mathrm{H}}_2\mathrm{S}}{\to }$

As Lewis basic strength decreases,

lone pair donation tendency decreases,

therefore stability of Lewis acid-base adduct decreases.