Analyzing the Context:
The sentence describes a situation where initial "rapid progress" was followed by a negative turn: "unfortunately, his injuries retarded the recovery of his muscular coordination." The word "retarded" here means delayed or slowed down. We need a word that similarly describes a negative impact on his ability to speak, leading to frustration. The word in the blank should be parallel in meaning to "retarded" or indicate a hindering/damaging effect.
- A. animated: Means full of life or vigor; lively. This is the opposite of the negative impact described.
- B. dispelled: Means to make (a doubt, feeling, or belief) disappear. This doesn't fit the context of an ability being affected by injuries.
- C. unsettled: Means causing to feel anxious or uneasy; disrupted or disturbed. While the patient might be unsettled, the injuries unsettling his ability to speak doesn't quite fit the direct negative impact on the ability itself.
- D. inhibited: Means to hinder, restrain, or prevent (an action or process). This fits perfectly, as the injuries hindered his ability to speak.
- E. impaired: Means weakened or damaged. This also fits perfectly, as the injuries damaged or weakened his ability to speak.
- F. augmented: Means made greater in size or value. This is the opposite of a negative impact.

Both inhibited and impaired accurately describe a negative effect on his ability to speak that would be parallel to "retarded" (for muscular coordination) and would lead to frustration. They both convey a sense of hindering, diminishing, or damaging.