Rome faced constant attack after the Pax Romana because its army?

A. was becoming less loyal to the emperor.

B. was becoming more loyal to the emperor.

C. had trouble paying mercenaries to fight.

D. had trouble finding mercenaries to fight

I think it was D. The Roman army was no longer composed of the disciplined troops that built the empire but instead relied on mercenaries. They were not loyal troops but fought for money and treasure. Apart from that was the lack of military leaders who were capable devising tactics to defend the Empire.