The atomic number of an element is equal to the number of
contained in one atom of that element.
O neutrons
O protons
O protons and neutrons
electrons

The answer is Protons.

The nucleus of an atom consists of protons and neutrons. Protons are positively charged species whereas neutrons do not have any charge.

The atomic number of an element is equal to the number of protons in the nucleus.

An element is the purest form of a substance. When divided, it results in a large number of identical atoms—assuming no isotopes are there. Atoms contain three main types of particles: protons, neutrons and electrons. The protons are positively charged particles, while the electrons are negatively charged particles. In comparison, neutrons do not have any charge. The nucleus consists of neutrons and protons, while the electrons are outside the nucleus. The number of protons and neutrons combined together are known as the mass number of the element.