Which particles make up the nucleus of an atom? a. protons and electrons b. neutrons and electrons c. protons only d. protons and neutrons e. neutrons only

The correct answer is option D, that is, protons and neutrons.  

The center of an atom is known as the nucleus, it comprises nucleons, that is, neutrons and protons, and is enveloped by the cloud of electrons. The nucleus exhibits the majority of the mass of the atom in spite of being a very small constituent of the atom. Approximately all the atom's mass is formed of the neutrons and protons in the nucleus with a very small donation from the orbiting electrons.