Which of the following factors may lead to an area being densely populated?

Lowland plains may lead to an area being densely populated. This is because lowland plains are usually flat and have fertile soils, which make them ideal for agriculture. As a result, people are attracted to these areas for farming and other economic activities, such as trading and manufacturing. In addition, lowland plains often have access to water sources such as rivers and lakes, which also support agriculture and other economic activities. These factors, combined with the ease of transportation and communication due to the flat terrain, make lowland plains attractive for settlement and development. Therefore, while high plateaux, rugged mountains, and active volcanoes may also have their own unique attractions and opportunities, lowland plains are more likely to be densely populated due to their suitability for agriculture and other economic activities.