Final answer:
Free radicals are molecules with an odd number of valence electrons, resulting in at least one unpaired electron. The correct answer to the question is 'a) All their electrons paired', as free radicals do not have all of their electrons in pairs, which contributes to their high reactivity.
Step-by-step explanation:
Free radicals are molecules that contain an odd number of electrons, resulting in at least one unpaired electron. Free radicals do not have all their electrons paired, which is what makes them highly reactive and unstable. Contrary to options b, c, and d in the student's question, free radicals do contain valence electrons, they often have an odd number of electrons, and they most certainly have an unpaired electron. Therefore, the correct answer to the question 'Free radicals do not have:' is 'a) All their electrons paired'.
For instance, nitric oxide (NO) is an example of a free radical; it has a total of 11 valence electrons, leading to an unpaired electron in its structure. This unpaired electron is what makes NO highly reactive and a characteristic free radical. Free radicals like NO readily participate in chemical reactions seeking to pair up that unpaired electron, thus gaining stability.