Final answer:
The correct answer is option C, which states that the presence of harriers excludes grosbeaks from the forest, aligning with the competitive exclusion principle in ecology.
Step-by-step explanation:
If harriers are in the forest, then grosbeaks are not. This implies the presence of one species excludes the presence of the other, suggesting that there's a competitive exclusion principle at play. When analyzing the options, option A) contradicts the premise by stating harriers and grosbeaks coexist. Option B) is also incorrect because it claims harriers and grosbeaks are never in the forest, which is not provided in the information. Option D) is false, as there's no indication that grosbeaks attract harriers. Therefore, option C) is correct, stating that the presence of harriers excludes grosbeaks aligns with the given statement and the principle of competitive exclusion, which suggests that two species that compete for the exact ecological niche cannot coexist if other ecological factors are constant.