Infectivity of chicken pox last for :: (A) Till the last scab falls off, (B) 6 days after onset of rash, (C) 3 days after onset of rash, (D) Till the fever subsides
Answer is B. “Period of communicability of varicella range from 1 to 2 days before the appearance of rash, and 4 to 5 days there after.”
Park 18/e, p 123
The patient ceases to be infectious once the lesions have crusted.
Scabs are not infective. So, option “a” is wrong.
Important features of chicken pox :

Causative Agent - Varicella zoster virus (HHV type 3)
Incubation period - 14 to 16 days
Secondary attack rate - 90%
Rash