Reason for Gas Becoming Plasma in Nuclear Fusion Reactor

In a nuclear fusion reactor, thermonuclear fusion requires nuclei to have sufficient kinetic energy to overcome their mutual electrostatic repulsion (Coulomb barrier) so that the attractive strong nuclear force can bind them together.

To achieve this, the core of the reactor is maintained at an extremely high temperature, typically on the order of $T \sim 10^7\text{ K}$ to $10^8\text{ K}$.

At such extremely elevated temperatures, the average thermal kinetic energy per particle is given by:
$E_{\text{thermal}} = \frac{3}{2} k_B T$

For $T \sim 10^7\text{ K}$, this thermal energy corresponds to:
$E_{\text{thermal}} \sim 10^3\text{ eV}$

which is vastly greater than the ionization energy of hydrogen isotopes (about $13.6\text{ eV}$). As a result of these energetic thermal collisions, electrons are stripped away completely from their parent nuclei (deuterons), transforming the neutral gas into a fully ionized collection of positive ions and free electrons, which constitutes the plasma state.

Therefore, the gas becomes plasma because of the high temperature maintained inside the reactor core.

Hence, the correct option is (D).