Final answer:
While the decline of the Roman West resulted from various factors, military invasions and external attacks are often considered pivotal due to their immediate and severe impact on the empire's ability to defend and govern its territories.
Step-by-step explanation:
The question concerns the administrative disintegration of the Roman West, a topic often debated by historians. Evaluating the most important factor is challenging as the decline was due to a complex interplay of various elements. However, many scholars consider military invasions and external attacks (b) as a crucial factor. These invasions significantly weakened the Western Roman Empire's ability to govern and defend its territories.
One should not overlook the economic instability and financial mismanagement (a), which led to the Empire's weakened state, making it more vulnerable to external threats. This situation was exacerbated as Rome began to lose large portions of the West, which drastically reduced tax revenues, further crippling their capacity to maintain military and administrative control.
Other factors, such as cultural shifts and societal transformations (c), and political corruption and leadership failures (d), contributed to the decline. The latter included factional politics that incited internal strife. Nonetheless, the immediate impacts of military pressure, particularly from Germanic tribes, were more visibly catastrophic and thus often regarded as the main catalyst for the fall of the Western Roman Empire.