The correct answer is option 'C' - imaginary. Let's break down the question and explain why this is the correct choice.

1. The oldest things that are mentioned
- This implies that we are looking for the oldest things that are mentioned in some context.

2. Factual
- Factual means something that is based on facts or reality. It refers to things that are known to be true and can be proven or verified.

3. Hypothetical
- Hypothetical means something that is based on a hypothesis or an assumption. It refers to things that are imagined or supposed but may not be true or proven.

4. Imaginary
- Imaginary means something that exists only in the imagination or fantasy. It refers to things that are not real or based on reality.

5. Realistic
- Realistic means something that is based on reality or resembles reality. It refers to things that are possible or believable in a real-life context.

Now, let's analyze each option and determine which one is the correct answer:

a) Factual: Oldest things mentioned may include historical events, ancient civilizations, or scientific discoveries. These are based on facts and can be verified. However, the question asks for the oldest things mentioned, so this option is not the correct answer.

b) Hypothetical: Oldest hypothetical things mentioned may include ancient myths, legends, or speculative theories. These are based on assumptions or hypotheses but may not be true or proven. However, the question asks for the oldest things mentioned, so this option is not the correct answer.

c) Imaginary: Oldest imaginary things mentioned may include ancient folklore, mythical creatures, or imaginary worlds. These are based purely on imagination and do not exist in reality. This option fits the criteria of the question, as it refers to the oldest things mentioned that are not real or based on reality. Therefore, option 'C' is the correct answer.

d) Realistic: Oldest realistic things mentioned may include ancient artifacts, historical figures, or natural landmarks. These are based on reality and resemble real-life objects or events. However, the question asks for the oldest things mentioned, so this option is not the correct answer.

In conclusion, the correct answer is option 'C' - imaginary. This option aligns with the question's requirement of identifying the oldest things mentioned that are not real or based on reality.