Concept:Food shortage occurs when the availability or production of food is reduced below what is needed.
Explanation:Drought means lack of rainfall, which destroys crops and lowers food production directly.
A low rate of reproduction reduces population size, so it reduces the demand for food rather than causing shortage.
Emigration also decreases population size, which lowers pressure on existing food supplies.
Topography refers to the physical features of land and does not directly cause food shortage.
Therefore, among the given options, drought is the ecological factor that directly results in food shortage.
Answer:A. Drought