Answer is C

x days (A and B work together) + 3 days (B alone worked) = 1

$x*(\frac{1}{14}+\frac{1}{21})+ 3*\frac{1}{21}=1$

On solving, we will get $5x = 36 \implies x =\frac{36}{5}$

Therefore, total number of days to complete the work = $\frac{36}{5}$ + 3 = **$\frac{51}{5}$**