What trend occurs to atomic radii from left to right across a period?

The atomic radii decrease from left to right across a period due to the increase in the positive charge of the nucleus as protons are added to the atom. As you move across a period on the periodic table, each successive element has an additional proton and an additional electron. While the electrons are added to the same principal energy level, the increased nuclear charge (the number of protons) attracts the electrons more strongly, pulling them closer to the nucleus.

This stronger attraction results in a smaller atomic radius because the outermost electrons are drawn nearer to the nucleus, leading to a decrease in size. Therefore, the trend across a period is a decrease in atomic radii, making the correct answer to the question C.