Infectivity of chicken pox last for :
“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
Till the last scab falls off
6 days after onset of rash
3 days after onset of rash
Till the fever subsides