To identify the element with the largest atomic radius among the given options.

Atomic radius generally decreases as you move from left to right across a period in the periodic table. This is because the number of protons (and thus the nuclear charge) increases, pulling the electrons closer to the nucleus, while the number of electron shells remains the same.

All the elements provided are in the same period (Period 3):

• Sodium (Na): Atomic number 11, Group 1
• Magnesium (Mg): Atomic number 12, Group 2
• Aluminum (Al): Atomic number 13, Group 13
• Chlorine (Cl): Atomic number 17, Group 17

Since all elements are in the same period, the element furthest to the left will have the largest atomic radius.
Arranging them from left to right in Period 3:
Na (Group 1) $\rightarrow$ Mg (Group 2) $\rightarrow$ Al (Group 13) $\rightarrow$ Cl (Group 17)

Sodium (Na) is the furthest to the left among these elements. Therefore, it will have the largest atomic radius.

The final answer is $\text{A}$.