The property that describes the ease with which an atom gives up an electron to form a positive ion

This is known as 'oxidation' in the redox reaction. Whenever an atom loses electrons, it's called oxidation, and when it gains electrons - it's known as 'reduction' (because an electron is a negative charge, so it's reducing the charge of the atom).

Ido not understand. 

The answer is b because a,c,and d are opinons