Which one of the following statements is correct?

An atom is composed of two distinct regions: a central nucleus and an outer region containing electrons. The atomic nucleus is a small, dense, positively charged core that contains protons and neutrons [c1][t1]. Protons carry a positive electric charge, while neutrons are electrically neutral [t3][t6]. Together, these particles are known as nucleons and account for more than 99.9% of the atom's mass [t3][t7]. In contrast, electrons are negatively charged subatomic particles that reside outside the nucleus, enshrouding it in a cloud or occupying specific electron shells [t2][t4][t8]. While the number of electrons equals the number of protons in a neutral atom to balance the charge, they do not reside within the nucleus itself [t1][t9]. Therefore, the nucleus specifically houses both protons and neutrons, making option 4 the only correct statement regarding the constituents of the atomic nucleus.

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Glossary > p. 100
2. [2] https://www.osti.gov/opennet/manhattan-project-history/Science/NuclearPhysics/nucleus.html
3. [3] https://www.energy.gov/science/doe-explainsnuclei
4. [4] https://www.energy.gov/science/doe-explainselectrons