"The equations of general relativity and quantum mechanics, when combined, begin to shake, rattle, and gush with steam like a decrepit automobile." - in this sentence the use of 'decrepit automobile' to describe the combination of general relativity and quantum mechanics implies that both of them do not work well when combined together. This is mostly denoted by the simile of 'decrepit automobile'. The author further explains that working together these two give 'nonesensical answers' to well-posed 'physical questions'. Option C best befits the context here. C is the correct answer.