Why did newly independent nations largely maintain the administrative boundaries drawn by colonial powers as their international borders?

The correct option is Maintaining legal continuity was paramount to avoid widespread administrative and geopolitical instability.

Here's why:

• Maintaining legal continuity was paramount to avoid widespread administrative and geopolitical instability. This is the most accurate reason. When newly independent nations emerged from colonial rule, adopting the existing administrative boundaries as international borders provided a framework for governance and prevented immediate widespread conflicts over territory. Attempting to redraw all borders based on ethnic, linguistic, or geographical lines would have led to immense administrative chaos, internal strife, and potential wars with neighboring states, which were also newly independent and fragile. This principle is often referred to as uti possidetis juris.
• They were based solely on natural, fixed features like mountains. This is incorrect. While some colonial borders followed natural features, many were drawn arbitrarily by colonial powers, often dividing communities or ignoring geographical realities, leading to many of the "imperfections" mentioned in the question.
• The new nations were bound by the Treaty of Westphalia to keep them. This is incorrect. The Treaty of Westphalia (1648) established principles of state sovereignty in Europe and is not directly relevant to the legal obligations of newly independent nations in the mid-20th century regarding colonial borders.
• Colonial borders were considered perfectly accurate and undisputed. This is incorrect. The question explicitly states "Despite their imperfections," indicating that these borders were often flawed and frequently disputed, leading to numerous conflicts in the post-colonial era.

The final answer is $\boxed{\text{Maintaining legal continuity was paramount to avoid widespread administrative and geopolitical instability.}}$