Why do noble gases have positive electron gain enthalpy?(A)- It is difficult to add an electron due to its small size.(B)- It is difficult to add an electron due to high electronegativity.(C)- It is difficult to add an electron due to stable configuration.(D)- It is difficult to add an electron due to high electron affinity.

Noble gases have positive electron gain enthalpy because of option (C) - "It is difficult to add an electron due to a stable configuration."

Noble gases, such as helium (He), neon (Ne), argon (Ar), etc., have a stable electron configuration with a full valence electron shell. For example, helium has 2 electrons in its outermost shell, which is the maximum capacity for that shell. Neon has 8 electrons in its outermost shell, which is also a full valence shell. These noble gases have achieved a highly stable electron configuration that resembles that of the nearest noble gas that precedes them in the periodic table.

Because these noble gases already have a full valence shell, they are not inclined to gain additional electrons. Adding an extra electron to their already stable configuration would require overcoming a significant energy barrier, resulting in a positive electron gain enthalpy.

The other options (A, B, and D) do not accurately describe the primary reason for noble gases having positive electron gain enthalpy. It's not primarily about the small size (A), high electronegativity (B), or electron affinity (D), but rather the stability of their existing electron configuration.