Step 1: Positive morality.
It refers to social rules enforced by opinion/social pressure (not by the State). Hence (a) is correct.
Step 2: Why others are incorrect.
(b) Legal and natural justice overlap; they are not disjoint.
(c) International law is sourced both from custom and conventions; calling it only “conventional” is inaccurate.
(d) False because (a) is correct.
\[
\boxed{(a)}
\]