The average atomic mass of a chlorine atom can be determined by considering the relative abundance of its isotopes. Chlorine has two isotopes: chlorine-35 and chlorine-37.

Option 1, 35-9 u, is not the correct answer because it is not a valid value for the average atomic mass of chlorine. The "-9 u" does not make sense in this context.

Option 2, 35-5 u, is the correct answer. This means that the average atomic mass of a chlorine atom is approximately 35.5 atomic mass units (u).

Option 3, 35.0 u, is not the correct answer because it does not take into account the relative abundance of the isotopes. The average atomic mass should be a value between the masses of the two isotopes.

Option 4, 37-0 u, is not the correct answer because it represents the mass of the chlorine-37 isotope alone, not the average atomic mass.

In summary, the correct answer is option 2, 35-5 u, which represents the average atomic mass of a chlorine atom.