Which of the following structures is characteristic of some bacteria and helps them survive extreme conditions?

The endospore is a highly resistant structure formed by some bacteria, which allows them to survive extreme environmental conditions, such as high temperatures, desiccation, and exposure to chemicals. This structural adaptation is a protective mechanism that ensures the bacterial cell can endure adverse situations until conditions become favorable for growth and reproduction again.

When formed, the endospore encapsulates the bacterial DNA and essential cellular components, creating a dormant state. This ability to withstand harsh environments is critical for the survival of bacteria in fluctuating conditions, such as in soil, where they might face drought or extreme heat.

In contrast, other structures like the capsule, flagellum, and pili do not provide the same level of resistance to extreme conditions. Capsules aid in bacterial adherence and protection from phagocytosis, flagella provide motility, and pili allow for attachment and genetic exchange between bacteria. However, these adaptations do not confer the same resilience to extreme environments as endospores do.