(A) turned over a new leaf = "changed for the better" (II) - This idiom means to start behaving in a better way, to reform oneself.
(B) set his face against = "sternly opposed" (IV) - This idiom means to be firmly and resolutely opposed to something.
(C) put me on my mettle = "roused me to do my best" (I) - This idiom means to challenge someone to prove their ability and do their utmost.
(D) to meet trouble half-way = "worry about the result before it's declared" (III) - This idiom means to worry unnecessarily about problems before they actually occur.