Answer:
Serious.
Step-by-step explanation:
Humor is funny; comical; amusing. There is no sign of joking or comedy.
Mysterious is being beyond one's powers to know, understand, or explain. We also don't see any sign of anything that we don't know of; suspicious.
Educational is Informative, instructive; factual. The passage is clearly not giving us any info that we don't already know.
Serious this tone in writing creates a level of suspense within the reader. It increases their focus because the concepts being offered are important.
Hope this helps :)
-ilovejiminssi