All isotopes of oxygen must have:

• A. the same number of protons
• B. the same number of electrons in the nucleus
• C. the same number of neutrons
• D. the same mass number

Answer: (A)

Verified step by step guidance

1. Recall that isotopes of an element have the same number of protons but differ in the number of neutrons.
2. Understand that the number of protons defines the element's identity; for oxygen, this number is always 8.
3. Recognize that electrons are not located in the nucleus, so the statement about electrons in the nucleus is incorrect.
4. Note that the mass number varies among isotopes because it is the sum of protons and neutrons, which changes with different neutron counts.
5. Conclude that all isotopes of oxygen must have the same number of protons, which is 8.