All atoms of the same element must have the same number if protons, however the number of neutrons can vary, giving different isotopes of the element.
I'm not quite sure what you were looking for but I hope that this helps.
The answer is 2. Magnetite. Hope I could help you :)
To claculate the gravitational attraction between two bodies with mass 1(m1) and mass 2 (m2) you need to use the equation:
F= G ((m1*m2)/r^2)
Where;
G is the gravitational constant (6.67E-11 m^3 s-2 Kg-1) and
r is the distance between the two objects.
Please check the attached picture
