Final answer:
According to the policy irrelevance proposition, C)only unanticipated policy actions can influence real GDP because individuals use all available information to forecast and adjust their behavior, rendering anticipated actions ineffective.
Step-by-step explanation:
The key implication of the policy irrelevance (policy ineffectiveness) proposition is that C. only unanticipated policy actions can influence real Gross Domestic Product (GDP).
According to the rational expectations hypothesis, individuals use all available information to predict future economic conditions and adjust their behavior accordingly.
Therefore, only policies that are not anticipated by the public—those that come as a surprise—can affect real GDP in the short run. Anticipated policy actions are already factored into economic agents' decisions and thus have no effect on real GDP.