The correct answer is $\boxed{\text{C}}$.
To find the determinant of a 3×3 matrix, we can use the formula:
$$\det \begin{bmatrix}
a & b & c \
d & e & f \
g & h & i
\end{bmatrix} = (aei – bg) + (bf – ch) + (cd – ae)$$
In this case, we have:
$$\begin{align}
\det \begin{bmatrix}
1 + b & b & 1 \
b & 1 + b & 1 \
2 & 2b & 1
\end{bmatrix} &= (1 + b)(1 + b) – b(1) + (1)(2b) – (b)(2) \
&= (1 + b)^2 – b^2 + 2b – 2b \
&= 2(1 – b)(1 + 2b) \
&= \boxed{2(1 – b)(1 + 2b)}
\end{align}$$
Option A is incorrect because $b$ does not appear in the determinant. Option B is incorrect because $b^2$ does not appear in the determinant. Option D is incorrect because $3b$ does not appear in the determinant.