Judging from their relative positions in the Periodic Table, which element in each set is more ...

(a) Lithium or potassium

In general, electronegativity increases from left to right across a row and from bottom to top of a column in the Periodic Table. This is because electronegativity increases with increasing positive charge on the nucleus and with decreasing distance of the valence electrons from the nucleus. Lithium is closer to the top of the Periodic Table and thus more electronegative than potassium.

(b) Nitrogen or phosphorus

Nitrogen is closer to the top of the Periodic Table and thus more electronegative than phosphorus.

(c) Carbon or silicon

Carbon is closer to the top of the Periodic Table and thus more electronegative than silicon.