Final answer:
To decrease post-operative edema and pain after an inguinal herniorrhaphy, the most effective nursing intervention is applying a scrotal support with an ice bag.
Step-by-step explanation:
The nursing intervention that is most appropriate to decrease post-operative edema and pain after an inguinal herniorrhaphy is d. applying a scrotal support with ice bag. This method helps by providing gentle compression, which can aid in reducing swelling and can also help manage pain through the numbing effects of the cold. In contrast, a truss is typically used pre-operatively for hernias, positioning for voiding won't directly reduce edema or pain, and while supporting the incision during coughing is beneficial for preventing incisional complications, it is not directly focused on reducing post-operative edema.