The correct option is
C Atomic radius
In a period, on moving from left to right, atomic number increases. In a
group, on moving from top to bottom, atomic number increases.
In a period, on moving from left to right, ionisation enthalpy increases. In a
group, on moving from top to bottom, ionisation enthalpy decreases.
In a period, on moving from left to right, atomic radius decreases. In a
group, on moving from top to bottom, atomic radius increases.
In a period, on moving from left to right, electron gain enthalpy increases. In a
group, on moving from top to bottom, electron gain enthalpy decreases.