Rapid population growth can lead to several challenges and costs. From the options you've mentioned, one significant cost is "poor sanitation."
As populations grow quickly, especially in urban areas, the infrastructure may not keep pace. This can result in inadequate sanitation facilities, leading to increased health risks, the spread of diseases, and overall public health crises. Poor sanitation is closely tied to issues such as waterborne diseases, which can further exacerbate infant mortality and overall health outcomes in the population.
Each of the other options—like infant mortality and seafood shortages—can also be influenced by rapid population growth, but poor sanitation often represents a direct consequence of insufficient infrastructure to support a larger population.