Final answer:
Internal strife within the Parthian Empire, notably due to feuding satraps and the desire for autonomy, along with nationalism resulting in revolts, were some of the significant reasons for rebellion. External threats, such as the rise of the more aggressive Sassanid dynasty, also played a role in the empire's challenges.
Step-by-step explanation:
One of the main reasons for internal rebellion in the Parthian Empire was feuding satraps (C) and another significant factor was nationalism spurring revolts and loss of territory (C). The Parthian governance system, which divided the empire into satrapies, with each governed by satraps, often led to power struggles within the empire. This was due to satraps operating with a fair degree of autonomy, sometimes even manipulating royal appointments to ensure a more pliable central authority that would allow them greater personal freedom. Also, the satraps' autonomy could lead to disparate policies and practices across the empire, occasionally fueling discontent among the local populations.
The pressures facing the Parthian Empire from within were further exacerbated by external challenges such as the ascendancy of the Sassanid dynasty, which was more aggressive and organized compared to its Parthian predecessors and posed a continuous threat, thus straining the empire's resources and cohesion.