The strongest oxidizing agent among the following is: A. Chlorine B. Iodine C. Fluorine D. Oxygen

The correct answer is $\boxed{\text{fluorine}}$.

Fluorine is the most electronegative element in the periodic table, meaning that it has the strongest attraction for electrons. This makes it a very strong oxidizing agent, meaning that it can easily remove electrons from other atoms.

Chlorine is also a strong oxidizing agent, but it is not as strong as fluorine. Iodine is a weaker oxidizing agent than chlorine, and oxygen is the weakest oxidizing agent of the four elements listed.

In general, the elements in the upper right corner of the periodic table are the strongest oxidizing agents. This is because they have the highest electronegativity and the lowest atomic number.