To provide effective control against muskrats, how much should the stone extend above the waterline?

The most suitable choice for effective control against muskrats is for the stone to extend two feet above the waterline. This height is significant because muskrats are agile swimmers and are adept at burrowing into banks, where they can create dens. Extending the stone at this height provides a physical barrier that deters them from establishing habitats near the water's edge.

A two-foot elevation offers sufficient protection without making it too challenging for the natural landscape, allowing for both stability and the maintenance of an effective deterrent. This approach ensures that muskrats are less likely to access the bank area easily, thereby reducing their populations and the potential for damage to aquatic vegetation and water quality.

While other options suggest greater heights, they may not proportionally enhance effectiveness and could be less practical or aesthetically pleasing in certain environments. Thus, two feet is considered both effective and reasonable in managing muskrat populations.