since "can ensure" is plural...wouldn't we need the plural noun "staff members" rather than the singular "large staff"? i was thinking B would be the answer...
can is used as both singular and plural. B is not plaralal.
"vast amounts of money" is a group so it is a singular where as "staff members" is just plural noun.
In D, "vast amounts of money" and "a large staff" both are groups.
So D is best.................