Question

What element has a mass number of 16 and 8 neutrons

Answer 1

The element would be Oxygen.
Work:
Mass = atomic # + neutrons
(The atomic number is the same as the number of protons in an element)

To find the number of protons substitute x 
16 = x + 8
x = 8 protons
Oxygen has 8 protons

I hope this helps!

Answer 2

The answer is O (oxygen).