If A is dependant, what does one call B?
A is dependent, it depends on B. If these were variables, B would be called an independent variable.
In programming, if A is a dependant of B, then B is a dependancy. This term is also fairly common in project and resource management.
However, in human relationships - if A is a child or spouse that is dependent on B, then A would remain a dependant but B would commonly be called a provider or maybe a supporter.
In project management (and lots of engineering), B is the prerequisite for A, in that it must come first. B does not necessarily know or care about A.