Which one of the following is not an isotope of hydrogen ?

Options

A. Protium
B. Ortho- para hydrogen
C. Deuterium
D. Tritium

Explanation

To solve the question "Which one of the following is not an isotope of hydrogen?", we need to understand what isotopes are and identify the isotopes of hydrogen.

### Step-by-Step Solution:

1. **Understand Isotopes**: Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. For hydrogen, the number of protons is always 1.

2. **Identify Isotopes of Hydrogen**: Hydrogen has three known isotopes:
- **Protium (¹H)**: This is the most common isotope of hydrogen, with 1 proton and 0 neutrons.
- **Deuterium (²H)**: This isotope has 1 proton and 1 neutron.
- **Tritium (³H)**: This isotope has 1 proton and 2 neutrons.

3. **List the Options**: The question likely provides several options, including:
- Protium
- Deuterium
- Tritium
- Ortho-hydrogen
- Para-hydrogen

4. **Identify Non-Isotopes**: Among the options, Protium, Deuterium, and Tritium are all isotopes of hydrogen. However, Ortho-hydrogen and Para-hydrogen are not isotopes; they are different spin states of the same hydrogen molecule (H₂).

5. **Conclusion**: The answer to the question is that **Ortho-hydrogen** and **Para-hydrogen** are not isotopes of hydrogen. If the question asks for one option, you can select either Ortho-hydrogen or Para-hydrogen.

### Final Answer:
**Ortho-hydrogen** (or Para-hydrogen) is not an isotope of hydrogen.