Incorrect.

If a is any multiple of 35, a can equal 70. But the GCD of 70 and 120 is 10 not 5 and this works for any multiple of 70. What is wrong?

@industriousbovine Hi there industriousbovine; thank you very much for your comment! Yes, indeed, this answer choice and its accompanying solution could be clarified a bit, because it meant to say that \(a\) can equal \(35p\) where \(p\) is not a common factor of \(168\) or \(120.\)
(In your example, we get \(70\) by multiplying \(2\) to \(35;\) however, \(2\) is another factor of \(120\) already, so we shouldn't allow it.)
You are right that the answer choice isn't explicit enough about this, and we will try to adjust the question make this more clear. Thank you so much for noticing this and letting us know; your help is really appreciated!

@debbie ok thank you