Which of the following is a mechanism of direct radiation interaction?

Direct radiation interaction refers to the immediate and direct impact that radiation has on atoms and molecules in a substance, typically resulting in ionization or excitation of electrons.

Excitation is the correct choice because it specifically describes the process by which a photon of radiation transfers energy to an electron in an atom, causing that electron to move from a lower energy level to a higher energy level within the atom. This excited state can lead to various effects within biological systems, including potential cellular damage, but the key aspect here is that this interaction involves the direct absorption of energy from the radiation by the atom itself, resulting in excitation.

The other options represent different concepts related to radiation effects but do not fit the definition of direct radiation interaction. Free radical formation involves indirect effects where radiation causes ionization of water molecules, leading to the production of free radicals that can cause secondary damage. Radical reactions pertain to the effects and interactions of these free radicals rather than direct radiation interactions with an atom's electrons. Cell repair refers to biological processes that occur after cellular damage has happened but is not a mechanism of direct interaction with radiation.