The keyword is ‘ MOST CRITICAL ‘ and the correct answer is ‘ Management Support ‘ as the management must be convinced of its necessity and that’s why a business case must be made. The decision of how a company should recover from any disaster is purely a business decision and should be treated as so.
The other answers are incorrect because :
Security policy is incorrect as it is not the MOST CRITICAL piece.
Availability of backup information processing facilities is incorrect as this comes once the organization has BCP Plans in place and for a BCP Plan , management support must be there.
Staff training comes after the plans are in place with the support from management. Reference : Shon Harris , AIO v3 , Chapter-9: Business Continuity Planning , Page : 697.