When does the use of economic rewards and penalties for controlling population work optimally? 

A)  when the strategies reinforce existing customs and trends 
B)  when the strategies are retroactive 
C)  when the strategies decrease a poor family's income or land 
D)  when the strategies push rather than nudge people to have fewer children

Economic rewards and penalties for controlling population work optimally when they reinforce existing customs and trends. This means that the strategies should align with the cultural norms and beliefs of the population. For example, if a community values education, providing economic incentives for families with fewer children who prioritize their education can be effective. However, if the strategies go against cultural norms or push people too aggressively, they may be less effective.