**Core Concept**
Chickenpox, caused by the varicella-zoster virus, is a highly contagious infection that spreads through respiratory droplets, direct contact with rash lesions, or airborne transmission. The infectivity period is a critical aspect of managing and preventing the spread of the disease.

**Why the Correct Answer is Right**
The infectivity of chickenpox typically begins 1-2 days before the onset of the rash and lasts until all the lesions have crusted over, usually around 5-7 days after the onset of the rash. During this period, the individual is most contagious when they have a high viral load, which is approximately 1-2 days before the appearance of the rash and continues until the lesions have fully crusted over.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the infectivity period of chickenpox does not start from the day of symptom onset. The virus is present in the individual's system for a day or two before the rash appears, making this option too early.
**Option B:** This option is incorrect because the infectivity period of chickenpox is not just limited to the duration of the rash. The individual remains contagious even after the lesions have crusted over, although the viral load starts to decrease.

**Clinical Pearl / High-Yield Fact**
It is essential to note that individuals with weakened immune systems, such as those with HIV/AIDS or undergoing chemotherapy, may remain contagious for a longer period, up to 5-7 days after the lesions have crusted over.

**Correct Answer: C. 5-7 days.**