Hello there!
Your answer would be D). strengthened ties between nations.
The reason why "D). strengthened ties between nations" would be the correct answer is because when countries trade with each other, it builds up a trust between them.
Trading between countries give each other a more stronger connection between them, due to the fact that they have to build trust up when they trade. When countries trade, it also allows them to call each other "allies" because they got another country or nation to depend on when something is not going so well in the economy, and they have someone to supply them with the supplies they need for their economy. "Trust" and "companionship" is what keeps countries closely connected and allows them to feel like they're part of a team.