Step 1: Relating kinetic and rest energy.
At the velocity \( v \), the kinetic energy is equal to the rest energy. This gives us the relation \( \frac{1}{2}mv^2 = mc^2 \), leading to \( v = c \sqrt{3}/2 \).
Step 2: Conclusion.
Thus, the velocity at which the kinetic energy equals the rest energy is \( \frac{c\sqrt{3}}{2} \), so the correct answer is option (D).
Final Answer:
\[
\boxed{\frac{c\sqrt{3}}{2}}
\]