Final answer:
Adaptive characteristics include a quicker response upon secondary pathogen exposure, varied pathogen receptors, and the ability to recognize broad classes of pathogens. Characteristic A, the slow initial response time, is not adaptive by itself. Hence, the adaptive characteristics are B, C, and D.
Step-by-step explanation:
The characteristics B, C, and D mentioned are adaptive in nature. Adaptive immunity is a part of the immune system that is highly specific to an invading pathogen and usually takes days or weeks to fully establish, as opposed to the innate immune system which responds immediately but non-specifically to pathogens.
Characteristic B, which is that the response responds more quickly upon secondary exposure to pathogens, is adaptive because it corresponds to immunological memory. After a primary infection, the body develops memory cells that enable a faster and stronger response upon subsequent exposures.
Characteristic C, that pathogen receptors are extremely varied, is adaptive because it ensures that the adaptive immune response can recognize and target a vast array of antigens.
Characteristic D, the ability to recognize broad classes of pathogens, is also adaptive because it allows the immune system to respond to many different types of pathogens, providing a broad defense against infections.
Characteristic A, on the other hand, is a description of the immune system's timing, not its adaptability. Therefore, it is not necessarily adaptive by itself.
Therefore, the correct answer to the student's question is E. B, C, and D.