The correct order will be ##"Na" < "Si" < "S" < "O"##

The correct order will be ##"Na" < "Si" < "S" < "O"##.

Because only three of the four listed are in the same period of the , you'll have to take into account both .

So, electronegativity (EN) increases when going from left to right across a period, and decreases when going down a group.

RIght off the bat, oxygen will be the most electronegative of the group because it's located in period 2, group 16, as opposed to the other three elements which are located in various groups in period 3.

For the other three elements, sodium will be the least electronegative of the group, since it's located in group 1, and sulfur will be the most electronegative, since it's located in group 16.

As a result, in order of increasing electronegativity, you'll get

##color(green)("Na" < "Si" < "S" < "O")##