Electronegativity tends to increase from left to right across a row, because a. the number of energy levels increases. b. the shielding effect increases. c. the attraction between the nucleus and the valence electrons increases. d. the valence electrons are located farther away from the nucleus.

C. the attraction between the nucleus and the valence electrons increases.

Explanation: Electronegativity is the tendency of the element to attract the shared pair of electrons towards itself.

Electronegativity of the elements increase across a period as the electrons get added to the same shell and the nuclear charge increases, the valence electrons become near to nucleus and thus the tendency of the elements to attract the shared pair of electrons itself increases.

Electronegativity of the elements decrease down a group as the electrons get added to new shell and the nuclear charge increases, but the effect of addition of shells is more dominant  and thus the size increases and the tendency of the elements to attract the shared pair of electrons itself decreases.