Why did "feudalism" develop after the fall of the Western Roman Empire?

a)Lack of a strong central authority and the constant threat of armed attack led to the establishment of a new institution.

b)The existence of a strong central authority increased the need to join wealthy landowners and vassals with large armies in light of the constant threat of armed attack from invaders.

c)Lack of a strong central authority resulted in the establishment of a new institution when political contracts were made between the monarchy and local leaders.

D)Lack of a strong central authority led to military and land alliances between wealthy landowners for economic advantages and protection during time of attacks. PLEASEEEE HELP ASAP