(A) Oxygen has highest tendency, among chalcogens, to form dinegative ion
(R) Electron affinity of oxygen is highest among chacogens

Chalcogens are the elements of Group 16 in the periodic table, including oxygen, sulfur, selenium, tellurium, and polonium. They have six valence electrons in their outermost shell and tend to form two negative ions.

Statement R is true because the electron affinity of oxygen is the highest among the chalcogens. Electron affinity is the energy change that occurs when an atom gains an electron to form an anion. Oxygen has a high electron affinity because it has a small atomic radius and a high effective nuclear charge, which means that it can attract an additional electron more strongly than other chalcogens.

Statement A is also true because the high electron affinity of oxygen makes it more likely to form a dinegative ion, O^2-. This is because oxygen can gain two electrons to fill its outermost shell and achieve a stable noble gas configuration. Sulfur, selenium, tellurium, and polonium can also form dinegative ions, but their electron affinities are lower than that of oxygen, so they are less likely to do so.