Let's break down the sentence and find the best fit for the blank:

Analyzing the Context:
The sentence states that John felt "unwell and was forced to take a break" due to the conditions "near the top of the peak." This implies that the conditions were physically demanding, perhaps causing dizziness, sickness, or extreme discomfort.
- A. vertiginous: Relating to or affected by vertigo; causing dizziness. Conditions near a peak, especially high altitude, can certainly cause dizziness. This fits the "felt unwell" perfectly.
- B. soporific: Tending to induce drowsiness or sleep. Feeling unwell and needing a break due to altitude isn't typically associated with sleepiness as the primary symptom of discomfort, but rather more acute physical distress.
- C. exacting: Making great demands on one's skill, attention, or other resources. While hiking a peak is exacting, the blank describes the conditions themselves that make one feel unwell, not necessarily the task being demanding.
- D. placid: Not easily upset or excited; calm and peaceful. This is the opposite of conditions that would make one feel unwell and need a break.
- E. excruciating: Intensely painful. While the unwell feeling might be intense, "excruciating" implies pain, which isn't the most direct or common symptom of high-altitude discomfort unless specifically stated. However, extreme physical discomfort can be described as excruciating.
- F. nauseating: Causing nausea or disgust. Feeling unwell near a peak is very commonly associated with nausea (mountain sickness). This is a strong fit.

Considering "felt unwell" and "forced to take a break," symptoms like dizziness and nausea are very common in high-altitude or peak conditions.

Both vertiginous (causing dizziness/vertigo) and nauseating (causing nausea) directly relate to feeling unwell in such an environment. "Excruciating" is also a possibility if the unwell feeling was intensely painful. However, "vertiginous" and "nauseating" are more specific and common descriptions of what happens at high altitudes or "near the top of the peak."

Let's re-evaluate "excruciating." If the conditions were intensely painful, that would certainly make one feel unwell. But the primary impact of "conditions near the top of the peak" often points to altitude sickness symptoms like dizziness and nausea first.

Between vertiginous and nauseating, they both clearly explain why someone would "feel unwell" at a high altitude.