**True strain :**

$\begin{align*} \epsilon_t = \sum \left ( \frac{\Delta L}{L} \right ) = \int_{L_i}^{L_f} \frac{dl}{l} = \ln \frac{L_f}{L_i} \end{align*}$

Now, here $L_f = 10 mm$ and $L_i = 20 mm$

So, $\begin{align*} \epsilon_t = \ln \frac{10}{20} = -0.6931 \end{align*}$

Answer **B**