Atomic weight and integer principle

[Awuah29]

What kind of questions is this? Anyone has clue to what its pertaining ?
The answer is 3, but why? I thought it was 1
thanks

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[BodybldgDoc]

I think it states the answer pretty clearly.
Every element is made up of different isotopes that contribute to its true atomic weight. This is why #3 is correct.

http://en.wikipedia.org/wiki/Isotope