The black circles in the center of the model represent neutrons. What do the white circles in the center represent?

electrons

nuclei

protons

atoms

The white circles represents the protons in the centre of the model which represents the nucleus of the atom.

Explanation:

There are three sub particles in the atom namely ''electrons, protons and neutrons''. The neutron and proton are in the centre while the nucleus of atom and electron are at the periphery. Electrons revolves around the nucleus and are represented farther from the centre.

In the centre, protons are represented as white circles and neutrons are represented as the black circles. The number of neutrons and protons are equal in the centre.

Electrons I believe is the answer.