Gregor Mendel is best known for his discoveries related to the inheritance of traits in living things. Specifically, he discovered how traits are passed from one generation to the next through his work with pea plants. His experiments laid the foundation for the field of genetics, where he formulated the laws of inheritance, including the concepts of dominant and recessive traits.
Therefore, the correct answer to what Mendel discovered about living things is "how traits are passed to the next generation."