If there is any undecidable problem and if we are able to convert it into some other problem then that problem will also be undecidable.
If it is already known that a problem is decidable & if we convert some other problem to decidable problem then that problem will also be decidable.
A <m (B) means A cannot be harder than B.
So option C is right.
A Reducible to B means every string of A is present in B. so something like this A is a subset of B.