Answer (A) is correct. Production flow through a constraint is managed using the drum-buffer-rope (DBR) system. The drum (i.e., the beat to which a production process marches) is the bottleneck operation. The constraint sets the pace for the entire process. The buffer is a minimal amount of work-in-process input to the drum that is maintained to ensure that it is always in operation. The rope is the sequence of activities preceding and including the bottleneck operation that must be coordinated to avoid inventory buildup.