The properties that decrease and increase down the group in a periodic table, respectively are:
(A) Electronegativity and electron gain enthalpy
(B) Electronegativity and atomic radius
(C) Atomic radius and electronegativity
(D) Electron gain enthalpy and electronegativity

Hint: Electronegativity is the tendency of an atom to attract a shared pair of electrons towards itself and decreases as we move down the group.

Complete step by step answer:
When we move down the group, the number of electron shells increases, leading to an increase in atomic size and there is lesser attraction towards the nucleus and the outermost shell. This leads to the decrease in electronegativity of an atom.
Atomic radius is the distance between the nucleus of an atom to its outer electron shell. As we move down the group, the number of electron shells increases and as the electron shell increases, the atomic radius increases as well.
Therefore, Option (B) is correct.

Note:
Electronegativity is a dimensionless property and can be measured using several scales and the most commonly used scale was designed by Linus Pauling. The most electronegative element is Fluorine with a value of $4.0$ and the least is Cesium with a value of $0.7$.
Metals are generally electropositive in nature and show lesser electronegativity as compared to non-metals.