In his breeding experiments, how did Mendel know which traits came from which pair of plants?

Crossing true breeding plant

Explanation:

Gregor Mendel is known as the Father or Genetics as he was the first successful person to observes and study and understand the pattern of inheritance of traits in the population for which he selected the Pea plants.

He performed the experiment by crossing the plants with different varieties of traits like the color of the flower, shape and structure of pods and others.

He knew that which plant is responsible for the trait of the progeny as he grew the pure-breeding plants which when were self-fertilized produced the same traits as of their parent and when cross-fertilized determined the trait of the progeny.

Thus, Crossing true breeding plant is the correct answer.