The correct answer is Nucleus. In everyday science, understanding the structure of an atom is important. The nucleus is the central part of the atom where two types of particles are found: protons and neutrons. Protons have a positive electric charge, while neutrons have no charge at all. These two stay tightly packed in the nucleus.

The nucleus is very small compared to the entire atom, but it contains almost all of the atom’s mass. Outside the nucleus, tiny negatively charged particles called electrons move around in circular paths called orbits or shells. These electrons are much lighter than protons and neutrons.

The number of protons in the nucleus tells us what element the atom belongs to. For example, if there is one proton, it is hydrogen; if there are eight, it is oxygen. The neutrons help in balancing the atom’s mass and provide stability to the nucleus.

In simple words, the nucleus is like the "core" or "heart" of the atom. It plays a key role in defining the atom and how it behaves in reactions. This makes the nucleus a very important concept in everyday science.