Final answer:
The nucleus is the small, dense, positively charged center of an atom, containing protons and neutrons, and is surrounded by negatively charged electrons in an electron cloud.
Step-by-step explanation:
The nucleus is the correct answer to the question, which is the small, dense center that has a positive charge and is surrounded by moving electrons. The nucleus is at the core of an atom, typically made up of both protons and neutrons. Protons have a positive electrical charge, whereas neutrons are electrically neutral. The electrons, which are negatively charged, orbit this central mass within an electron cloud, balancing out the atomic charge as there is an equal number of protons and electrons in a neutral atom.
The nucleus contains practically all the mass of an atom, making it extremely dense. To put this into perspective, if an atom were the size of a football stadium, the nucleus would be about the size of a pea, yet it holds nearly all the atom's weight.