Explanation

(c)
Any month out of 12 months can be chosen with probability $\frac{1}{12}$
There are 7possiblle ways, in which the month can start and it will be a Sunday on 6th day, if the first day of the month is Tuesday, whose probability is $\frac{1}{7}$
$\therefore$ Required probability is $\frac{1}{7}$
$\therefore$ Required probability$= \frac{1}{12} \times \frac{1}{7} = \frac{1}{84}$