Final answer:
Singapore has the highest population density among the given Southeast Asian countries due to its small land area and high level of urbanization.
Step-by-step explanation:
Among the Southeast Asian countries listed, Singapore has the highest population density. This is because Singapore is a highly urbanized and small island nation with limited land area and a relatively large population for its size. For comparison, Indonesia has a much larger population overall, but it is spread across many islands, including the densely populated island of Java. The Philippines also has a significant population concentrated on its islands, such as Luzon, but again spread over a larger area than Singapore. While Cambodia's population is lower than the others and spread across its land area, making it less dense.