**Core Concept**
Chickenpox, also known as varicella, is a highly contagious viral infection caused by the varicella-zoster virus. Secondary attack rate refers to the proportion of exposed individuals who develop the disease.

**Why the Correct Answer is Right**
The secondary attack rate of chickenpox is typically high, ranging from 90% to 100%. This is because the virus is highly contagious and is spread through respiratory droplets, direct contact with rash lesions, and airborne transmission. The high secondary attack rate is due to the virus's ability to infect individuals who have not developed immunity, such as those who have not previously been infected or vaccinated.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the secondary attack rate of chickenpox is not typically lower than 50%.
* **Option B:** This option is incorrect because the secondary attack rate of chickenpox is not typically lower than 70%.
* **Option C:** This option is incorrect because the secondary attack rate of chickenpox is not typically lower than 80%.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the secondary attack rate of chickenpox can be reduced through vaccination, which provides immunity to the varicella-zoster virus. Vaccination is particularly important for individuals who are at high risk of complications, such as young children, pregnant women, and those with weakened immune systems.

**Correct Answer: D. 90-100% (Note: Correct answer was missing, assuming the correct answer is D. 90-100% for the sake of the explanation)**