What symptom may indicate that a person is progressing through intoxication?

C is the correct choice because symptoms such as nausea and blurred vision are commonly associated with the progression of intoxication. As a person becomes more intoxicated, their body may react negatively to the presence of alcohol or other substances, leading to feelings of nausea. Additionally, blurred vision is a well-documented effect that occurs as the central nervous system is affected by elevated blood alcohol levels, impairing visual acuity and coordination. Both of these symptoms are indicative of intoxication severity and signal the body's adverse reaction to the substance.

Clear thinking and rational behavior reflect a state of cognitive functioning that is typically diminished under intoxication, while normal motor skills suggest no impairment, which contrasts what one would expect to see as intoxication progresses. Sharp eyesight does not align with the common physiological effects of intoxication, as it is generally diminished. Thus, both of these options do not accurately describe the symptoms that indicate a person is moving through the stages of intoxication.