Final answer:
Protectionist trade policies and socio-economic challenges are the most significant barriers to foreign trade and political relations, with issues like threats of 'unfair' trade practices and legislation adding restrictions being common.
Step-by-step explanation:
The most significant barrier to foreign trade and political relations between nations, as mentioned in the excerpt, appears to be protectionist trade policies. These policies are often enacted by countries to protect their domestic industries from overseas competition but result in more restrictions on trade. Factors such as threats to challenge 'unfair' trading practices, disputes resolution cases in the WTO and other trade agreements, legislation influenced by lobbyists, and street protests against trade rules are indicative of this barrier to trade. Additionally, socio-economic factors like rapid population growth, limited resources, political obstacles, and scarcity of development finance can hinder economic development and contribute to trade barriers.