Explanation:

- Introspection: This means examining one's own thoughts and feelings. It's more about self-reflection than looking at past events.

- Inspection: This refers to closely examining something or someone. It doesn't directly relate to looking back in time.

- Circumspection: It means being cautious and considering all circumstances before acting. It's about careful observation, not specific to past events.

- Retrospection: This is the act of looking back and thinking about the past. It directly matches the given phrase, "the act of looking back on past time."

- Correct Answer: Option 4 - Retrospection