Option 1 -> coherent - means logical, clear, and well-organized, fitting perfectly with making complex ideas understandable.
Option 2 -> perplexing - means confusing or puzzling, which contradicts making concepts "easy to understand."
Option 3 -> vague - means unclear or imprecise, which wouldn't help make complex concepts understandable.
Option 4 -> superficial - means shallow or lacking depth, which doesn't align with effectively explaining complex concepts.
Hence, Option 1: coherent -> A coherent explanation is logical, clear, and well-structured, which directly supports the ability to make complex concepts easy to grasp. The sentence requires a positive quality that enables understanding, not one that creates confusion or lacks clarity. -> correct