Which of the following would likely lead to a decrease in population size in density-dependent factors?

Density-dependent factors are those that have a greater impact on a population as its density increases. Increased disease transmission is a prime example of a density-dependent factor because as the population becomes more crowded, individuals are in closer proximity to one another, facilitating the spread of diseases. This can result in higher mortality rates or reduced reproduction, leading to a decrease in population size.

In contrast, a natural disaster typically affects populations regardless of their density and is classified as a density-independent factor. An increase in available resources, such as food and habitat, would support population growth rather than contribute to a decline. Lastly, more favorable environmental conditions can enhance survival and reproduction rates, again leading to an increase in population size. Thus, the correct choice illustrates a clear mechanism through which population size can decrease due to the effects of disease spread in a densely populated area.