The most electronegative element of period-3 is: A. Ar
B. S
C. Si
D. Cl

The most electronegative element of period 3 is chlorine (Cl).

Explanation:
Electronegativity is the ability of an atom to attract shared electrons in a chemical bond.
As we move across a period from left to right in the periodic table, electronegativity generally increases because the number of protons in the nucleus increases, pulling electrons closer.
In period 3, the elements are:
Na, Mg, Al, Si, P, S, Cl, Ar

Argon (Ar) is a noble gas, and noble gases typically do not form bonds easily since they have a full outer electron shell, so they are not considered electronegative.

Silicon (Si) and sulfur (S) have lower electronegativities compared to chlorine (Cl).

Chlorine (Cl), being near the right end of the period, has a very high tendency to attract electrons because it has a nearly full valence shell (7 valence electrons). Hence, it is the most electronegative element in period 3.

Conclusion:
The most electronegative element in period 3 is chlorine (Cl).
Answer: D. Cl