WordNet sense 1 (a type of security issued by a corporation (usually together with a bond or preferred stock) that gives the holder the right to purchase a certain amount of common stock at a stated price):

stock warrant, stock-purchase warrant

WordNet sense 2 (formal and explicit approval):

countenance, endorsement, imprimatur, sanction, indorsement

WordNet sense 3 (a written assurance that some product or service will be provided or will meet certain specifications):

guarantee, warranty, warrantee

WordNet sense 4 (show to be reasonable or provide adequate ground for):

justify