A. Oxygen, atomic number 8
B. Fluorine, atomic number 19
C. Helium, atomic number 2
D. Fluorine, atomic number 9
Fluorine has a high electronegativity, and therefore, it has a strong attraction for electrons and tends to gain electrons to form anions. Therefore, it is more likely to become a cation by losing an electron.
Therefore, the Correct Answer is D.