#### Answer

b.

#### Work Step by Step

$\displaystyle \lim_{x\rightarrow 4^{-}}f(x)=\lim_{x\rightarrow 4^{+}}f(x)=6,\ \ \ \ $ so
both the limits from the left and from the right exist,
and, they are equal,
which means that $\displaystyle \lim_{x\rightarrow 4}f(x)$ exists and
$\displaystyle \lim_{x\rightarrow 4}f(x)=6$.