Final answer:
The true statement about all Mainland Southeast Asia countries is not provided in the options. They are neither all small nor under Chinese rule, and their wealth varies greatly.
Step-by-step explanation:
The statement that is true about all the countries in Mainland Southeast Asia is not listed among the provided options.
Mainland Southeast Asia, which includes Vietnam, Laos, Cambodia, Thailand, and Myanmar, presents a diverse range of countries with different sizes, economic statuses, and political histories.
None of the countries in Mainland Southeast Asia are under the rule of China, they vary in size, and their economic wealth is diverse, with some nations being more economically developed than others.