Final answer:
The element with the largest atomic radius among Na, K, Mg, and Ca is K (Potassium), as atomic radius increases down a group in the periodic table and K is located below Na in the alkali metals group.
Step-by-step explanation:
To determine which element contains atoms with the largest radius from the given options (Na, K, Mg, Ca), we refer to trends in the periodic table. Atomic radius generally increases down a group and decreases across a period from left to right. Thus, within the same group, elements towards the bottom have larger atomic radii due to the addition of electron shells. Comparing the given elements: Na (Sodium) and K (Potassium) are in the same group (alkali metals), with potassium being below sodium, so K has a larger radius.
Mg (Magnesium) and Ca (Calcium) are in another group (alkaline earth metals), with calcium below magnesium, so Ca has a larger radius. Between K and Ca, which are in different groups but the same period, K has the largest atomic radius because it is in the group with typically larger atomic radii (alkali metals). Therefore, the answer is B. K (Potassium).