Which of the following would make a sledge unusable?

A rounded head on a sledge would render it unusable because the design of a sledge typically requires a flat, solid striking surface that can generate maximum impact when hitting a target. A flat head provides the necessary surface area to effectively transfer force to the object being struck, whether it's for driving a stake into the ground or breaking something apart. When the head is rounded, it lacks the proper contact area, which reduces its effectiveness in delivering force and may lead to slipping or glancing off the target, undermining the tool's intended function.

In contrast, the other features, such as a flat handle, long handle, and heavy weight, can often be advantageous for different applications of a sledge. A flat handle provides a comfortable grip, a long handle can increase leverage, and a heavier weight generally enhances the tool's striking power—each contributing positively to its effectiveness when used correctly.