What did mendel's cross-pollination of pea plants prove?

a. each parent only passes on one-half of its hereditary factors to the offspring.

b. it takes four generations of offspring to determine the parents' factors.

c. there is no such thing as a true-breeding plant; they all have hidden factors.

d. inherited factors combine in offspring to make entirely new characteristics.

The correct answer would be A. Each parent only passes on one-half of its hereditary factors to the offspring.

It can be correctly explained with the help of the law of segregation.  

It states that the alleles of each gene get segregated from each other and each gamete produced by a parent would contain only one out of two alleles of a gene.

For example, if the genotype of a parent is Tt, then the gamete would contain either T or t.

Thus, each parent only contributes half of its heredity factors in the form of gametes.