Which is a characteristic of the part of the atom marked 'a', assuming 'a' refers to the nucleus?

• A. It is surrounded by electrons in fixed orbits.
• B. It has a negative charge.
• C. It occupies most of the atom's volume.
• D. It contains protons and neutrons.

Answer: (D)

Verified step by step guidance

1. Identify what the nucleus of an atom is: the central part of the atom containing protons and neutrons.
2. Recall that protons have a positive charge, neutrons have no charge, and electrons (which orbit the nucleus) have a negative charge.
3. Understand that the nucleus is very small compared to the entire atom but contains almost all of the atom's mass.
4. Recognize that electrons are not in fixed orbits but rather exist in regions called orbitals around the nucleus.
5. Conclude that the nucleus contains protons and neutrons, has a positive charge overall, and occupies a very small volume relative to the atom.