Final answer:
In the given context, 'fatigue' means exhaustion, which is indicative of extreme tiredness from physical or mental effort.
Step-by-step explanation:
In the sentence 'By her position, leaning up against the trunk of the tree, I'd guess Glimmer was supposed to be on guard, but fatigue overcame her,' the word fatigue means B. Exhaustion. This is evident from the context given in various examples where characters are described as being worn out, having a lack of energy, or resting after feeling fatigued. This usage aligns with the definition of fatigue as a state of extreme tiredness resulting from mental or physical exertion or illness.