**Core Concept:** The secondary attack rate of chickenpox refers to the proportion of susceptible individuals within a population who develop the disease after exposure to an infected person. Chickenpox is caused by the varicella-zoster virus (VZV).

**Why the Correct Answer is Right:** The correct answer is **D**: A secondary attack rate of 30-40% indicates that approximately 30-40% of susceptible people in a population will develop chickenpox after being exposed to an infected person. This is due to the fact that most individuals have been previously exposed to the virus through natural infection or vaccination, rendering them immune. Consequently, the remaining susceptible individuals are at risk for infection upon exposure.

**Why Each Wrong Option is Incorrect:**

A. **100%**: This is incorrect because the correct answer demonstrates a range of 30-40%. A 100% secondary attack rate would mean that every susceptible individual would definitely develop chickenpox after exposure, which is not true.

B. **0%**: This is incorrect as well, as it contradicts the correct answer's range of 30-40%. A 0% secondary attack rate would mean no susceptible individuals exist, which is not the case.

C. **50%**: This option is wrong because it does not represent the correct range of 30-40%. A 50% secondary attack rate would mean half of the susceptible individuals develop chickenpox, which is not the correct answer.

**Clinical Pearl:** The concept of secondary attack rate is important when assessing the risk of transmission in a population. In the context of chickenpox, understanding this rate helps healthcare professionals and public health officials make informed decisions regarding disease control measures and vaccination strategies.