(a) f(2.56, 42) is illegal. The call is ambiguous, because either f(int, int) or f(double, double) is a better match than the other on one of the arguments to the call.
(b) f(42) has a best match function f(int).
(c) f(42, 0) has a best match function f(int, int).
(d) f(2.56, 3.14) has a best match function f(double, double).