Final answer:
The military development of the 'Five Good Emperors' that would later prove fateful for the Roman Empire was the switch from offensive operations to stationary frontiers.
Step-by-step explanation:
The military development of the 'Five Good Emperors' that would later prove fateful for the empire was A) the switch from offensive operations to stationary frontiers.
This change in military strategy led to a decline in the Roman Empire's ability to expand and conquer new territories, which ultimately contributed to its decline and fall. Instead of actively seeking to expand their borders, the 'Five Good Emperors' focused on maintaining their existing frontiers and defending the empire from external threats.
For example, this change in strategy can be seen in Emperor Trajan's decision to abandon his conquest of Mesopotamia and establish fixed boundaries along the Rhine and Danube rivers. While this decision helped to stabilize the empire in the short term, it also limited its ability to expand and acquire new resources and territories.