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Answer:
C. Number of outer shell electrons.
Step-by-step explanation:
Groups consist of the vertical columns of the periodic table, such as 1A, 2A, etc. (Not including transition metal groups) which depict the number of valence electrons, or the number of electrons in the atom's outer shell.
For example, Oxygen, which is in group 6A, contains 6 valence (outer shell) electrons.