Mendel knew that the variations in the offspring
generations resulted from his experiments
because he
•a. allowed plants to cross-pollinate.
.b. ensured that plants self-pollinated.
.c. controlled the fertilization process.
.d. changed the growing conditions.

controlled the fertilization process.

Explanation:

Mendel took precautions to avoid contamination by pollen through self pollination of the P generation. He controlled the fertilization processes between the plants. He self pollinated the first pureline varieties to form the P generation. He then cross pollinated them to get F1 generation. Every step of his experiment was controlled.

showing a photograph would make this a hell of alot easier