CBSE Class 12-science Answered

The attractive force is due to the nuclear force which is different from the coulomb force.

To further elaborate I have given the following information from wikipedia.

At much smaller separations between nucleons the force is very powerfully repulsive, which keeps the nucleons at a certain average separation. Beyond about 1.7 femtometer (fm) separation, the force drops to negligibly small values.

At short distances, the nuclear force is stronger than the Coulomb force; it can overcome the Coulomb repulsion of protons inside the nucleus. However, the Coulomb force between protons has a much larger range and becomes the only significant force between protons when their separation exceeds about 2.5 fm.

The nuclear force is nearly independent of whether the nucleons are neutrons or protons.