Explanation:

- The passage discusses imagination as a concept. When the text refers to "imagination," it's using a singular noun.

- "Its" is the appropriate possessive pronoun to use with singular nouns like "imagination."

- Using "its" aligns with the singular subject, indicating possession or characteristic of imagination.

- "His" and "their" would imply ownership by individuals or a group, which doesn't fit.

- "Our" implies collective ownership by people, which is incorrect since the focus is on imagination itself.

Given this understanding:

- The correct answer is "its".