Which reason best explains why bacteria is good at causing infections in other organisms? Bacteria have a rigid, protective layer that is called a pili. Bacteria are transmitted easily and reproduce quickly. Bacteria evolve slowly as they pass on genetic information. Bacteria are easily killed by a host's immune system or antibiotics.

A. Bacteria have a rigid, protective layer that is called a pili.

This doesn't actually say anything about its infectivity - it merely is saying that the bacteria may be harder to take out by other organisms in the immune system. While this indirectly relates, it does not relate enough to be the correct answer.

B. Bacteria are transmitted easily and reproduce quickly.

This directly answers the question.

C. Bacteria evolve slowly as they pass on genetic information.

This answer is somewhat of a red herring - it does not actually relate to the base question. Their ability to evolve has nothing to do with their ability to cause infections, except by a long stretch.

D. Bacteria are easily killed by a host's immune system or antibiotics.

This answer does not satisfy "why bacteria is good" and therefore is not a viable answer.