The correct answer is: A. main canal.
The outlet discharge factor is the duty at the head of the main canal. The duty is the amount of water that is delivered to a unit area of land. The outlet discharge factor is a measure of the efficiency of the irrigation system. A higher outlet discharge factor indicates that the system is more efficient in delivering water to the fields.
A branch canal is a secondary canal that branches off from the main canal. A watercourse is a natural or artificial channel that carries water. A distributory is a canal that distributes water from the main canal to the fields.
The outlet discharge factor is not the duty at the head of a branch canal, a watercourse, or a distributory. The duty at the head of a branch canal, a watercourse, or a distributory is the amount of water that is delivered to the branch canal, watercourse, or distributory from the main canal. The outlet discharge factor is a measure of the efficiency of the irrigation system, not the amount of water that is delivered to a unit area of land.