Atomic radius is the distance from the nucleus to the outermost electron shell, and its variation across a period is governed by changes in nuclear charge and shell number. Across the second period, electrons are added to the same principal shell while protons are added to the nucleus, so the nuclear charge increases without a corresponding rise in the number of inner shielding shells. The greater effective nuclear charge pulls the outer electrons closer, so the atomic radius decreases steadily from lithium to fluorine. The option of increase due to added shells is wrong, because no new shell is added within a period. The option of constancy ignores the changing nuclear pull. The option of increase then decrease describes neither a period nor a group trend correctly. This contraction across a period is a fundamental NCERT periodicity trend. Plausibility check: the radius of lithium is much larger than that of fluorine, confirming the decrease driven by rising effective nuclear charge.