Nonpathogenic bacteria are:

• A. able to survive only in extreme environments
• B. incapable of causing disease in their host
• C. responsible for most bacterial infections
• D. always beneficial to humans

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the term 'nonpathogenic bacteria'. These are bacteria that do not cause disease in their host organism.
2. Step 2: Review the options given: (a) able to survive only in extreme environments, (b) incapable of causing disease in their host, (c) responsible for most bacterial infections, (d) always beneficial to humans.
3. Step 3: Eliminate options that contradict the definition of nonpathogenic bacteria. For example, if bacteria cause disease, they are pathogenic, so option (c) is incorrect.
4. Step 4: Consider that nonpathogenic bacteria are not necessarily always beneficial; some may be neutral, so option (d) is not always true.
5. Step 5: Recognize that nonpathogenic bacteria are not restricted to extreme environments, so option (a) is incorrect. Therefore, the correct understanding is that nonpathogenic bacteria are 'incapable of causing disease in their host'.