Question

Why did Mendel prevent his plants from self-pollinating? so they couldn't produce offspring to insure that each plant's offspring would remain identical to the parents to control the crossing of the traits to maintain a pure strain

Answer 1

The first option. So they​ couldn't produce offspring.
Mendel was cross pollinating the plants. While he was doing to that he found out about "The Principal Of Dominance" meaning some alleles are recessive while some are dominant.