Final answer:
The three walls built around Alesia served the purpose of defense, supply, and observation.
Step-by-step explanation:
The correct answer is b) Three walls for defense, supply, and observation.
The purpose of the three walls built around Alesia was for defense, supply, and observation. These walls provided protection against invaders and allowed soldiers to keep watch and monitor enemy movement from a distance.
The walls were manned by a large number of soldiers and supported by garrisons, roads, beacon stations, couriers, and postal systems for communication.