Therefore, $E[M(t)] = E[|W(t)|]$.
Since $W(t) \sim N(0, t)$, the probability density function of $W(t)$ is given by
$$f_{W(t)}(x) = \frac{1}{\sqrt{2\pi t}} e^{-\frac{x^2}{2t}}$$
Then,
$$E[|W(t)|] = \int_{-\infty}^{\infty} |x| f_{W(t)}(x) dx =
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