Mendel obtained his p generation by allowing the plants to

self polinate

Explanation:

Mendel is considered as the father of genetics and he was famous for his experiments on pea plant and on the basis of his experiment on pea plants he postulated the principles of inheritance which is called Mendel's law.  

He used pea plant because pea plant can self pollinate, has bisexual flower, short life cycle, well discrete characters. So Mendel allowed the hybrid pea plant to self replicate so that the trait that is hidden can reappear in the second generation. So the right answer is self-pollination.

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