 Solution

Condition (b) immediately implies (c): a positive real multiple $tA$ of an ample divisor is an ample real divisor, and real linear equivalence of divisors implies numerical equivalence of divisors.

More explicitly, the real numerical class of $D$ is the sum of a class in the ample cone and an effective real divisor class. This separates strict positivity from the possibly degenerate effective part; it does not claim that the effective part itself is ample.