Final answer:
The irrelevant sentence is option c) "The main character's journey takes place in a magical realm."
Step-by-step explanation:
The sentence that is irrelevant to the central idea of the excerpt is option c) "The main character's journey takes place in a magical realm." This sentence does not relate to the central idea of the excerpt, which is about climate change and its impact on marine life. The other sentences discuss photosynthesis, the history of computers, and the impact of climate change on marine life, which are all relevant to the central idea.