The correct answer is option C: F, Cl, Br.
Atomic radius is the measure of the size of an atom, defined as the distance from the nucleus to the outermost electron shell. Generally, atomic radius decreases across a period and increases down a group in the periodic table.
Now, let's analyze the provided options:
A. K (Potassium), Ca (Calcium), Sc (Scandium): These elements are in the same period (period 4). As you move from left to right across a period, atomic radius generally decreases due to increasing effective nuclear charge. Therefore, the atomic radius order is Ca > K > Sc.
B. K (Potassium), Na (Sodium), Li (Lithium): These elements are in the same group (Group 1). Atomic radius generally increases down a group, so the order is Li < Na < K.
C. F (Fluorine), Cl (Chlorine), Br (Bromine): These elements are in the same period (period 3). As you move from left to right across a period, atomic radius generally decreases. Therefore, the order is Br > Cl > F.
D. Ge (Germanium), P (Phosphorus), O (Oxygen): These elements are in the same group (Group 16). Atomic radius generally increases down a group, so the order is O < P < Ge.
Therefore, the list of elements arranged from smallest to largest atomic radius is option C: F, Cl, Br.