Which condition makes a chisel unsuitable for use?

A chisel is primarily used for cutting, shaping, or carving materials, and its effectiveness directly relates to its physical condition. A mushroomed head signifies that the chisel has experienced excessive impact, causing the metal to deform and creating a bulging shape at the striking end. This deformation not only renders the chisel ineffective for precise cutting but can also lead to safety hazards, as the irregular shape can cause the chisel to slip or become uncontrollable during use.

In contrast, having a sharp edge is essential for a chisel to perform its intended function effectively. An unpolished tip might slightly reduce its efficiency but does not compromise its overall usability in the same significant manner as a mushroomed head. A rusted surface may affect aesthetic and minor effectiveness but can often be restored through cleaning. Thus, the condition that most negatively impacts a chisel's usability is the mushroomed head, rendering it unsuitable for proper tasks.