What does an endospore stain indicate about a bacterium?

The correct answer is significant because an endospore stain specifically highlights the ability of a bacterium to form endospores, which are highly resistant structures produced by certain bacteria in response to environmental stress. The formation of endospores is a survival mechanism that allows the bacterium to withstand extreme conditions such as heat, desiccation, and chemical damage. When the dye penetrates the endospore, it provides a clear visual indication that the bacterium possesses this remarkable capability.

Other options relate to different characteristics of bacteria but do not directly connect to the endospore staining process. For instance, the presence of toxins or pathogenicity is not determined through an endospore stain, nor does it provide insights into the bacterium's Gram staining characteristics. Thus, the focus on the specific ability to form spores makes the correct answer particularly relevant in understanding bacterial survival strategies in microbiology.