Unbiased estimator under a normal mixture

Let $X_1,X_2,…,X_n$ be a random sample from a continuous distribution with the probability density function

$$f(x)=\frac{1}{2\sqrt{2\pi}}\left[e^{-\frac12(x-2\mu)^2}+e^{-\frac12(x-4\mu)^2}\right],\quad -\infty<x<\infty.$$

If $T=X_1+X_2+\cdots+X_n$, then which one of the following is an unbiased estimator of $\mu$?