The tone of a factual passage is typically serious and objective. This means that the author presents information in a straightforward and unbiased manner, without trying to sway the reader's emotions or opinions. Let's break down this answer in detail:

Serious Tone:
- A factual passage is meant to provide accurate information, so the tone should be serious to convey the importance and gravity of the subject matter.
- The author avoids using humor or lighthearted language that could undermine the seriousness of the topic.

Objective Tone:
- Objectivity is a key aspect of a factual passage. It means that the author presents information without personal bias or emotional influence.
- The author relies on evidence, facts, and logical reasoning to support their claims, rather than trying to persuade the reader to agree with a particular viewpoint.
- By maintaining an objective tone, the author allows the reader to form their own opinions based on the presented information.

Why not the other options?
- Emotional and persuasive: Factual passages are not meant to evoke strong emotions or convince the reader to take a specific stance. Instead, they focus on presenting information accurately and objectively.
- Humorous and lighthearted: Factual passages typically deal with serious or important topics, and using humor or lighthearted language could undermine the gravity of the subject matter.
- Sarcastic and critical: Factual passages aim to inform and educate, not to criticize or mock. Using sarcasm or a critical tone would detract from the objective nature of the passage.

In conclusion, the tone of a factual passage is serious and objective. This allows the author to present information without personal bias, enabling the reader to make their own informed judgments based on the presented facts.