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

(a) Carbon or nitrogen
In general, electronegativity increases from left to right across a row (period) 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. Nitrogen is farther to the right than carbon in Period 2 of the Periodic Table. Thus, nitrogen is more electronegative than carbon.

(b) Chlorine or bromine
Chlorine is higher up than bromine in column 7A of the Periodic Table. Thus, chlorine is more electronegative than bromine.
(c) Oxygen or sulfur
Oxygen is higher up than sulfur in column 6A of the Periodic Table. Thus, oxygen is more electronegative than sulfur.