Which one of the following molecular hydrides acts as a Lewis acid?

${\mathrm{B}}_2{\mathrm{H}}_6$ is a Lewis acid here as it is electron deficient (6 electron) so it can accept electrons. $\mathrm{N}{\mathrm{H}}_3,{\mathrm{H}}_2\mathrm{O}$ are Lewis bases as they have lone pair of electrons to donate. $\mathrm{C}{\mathrm{H}}_4$ is neither Lewis acid nor Lewis base as it can not donate (have no lone pair) or accept electron (have no vacant orbital).