Final answer:
Mendel's law of independent assortment is explained by the random alignment of chromosomes during metaphase I of meiosis I, leading to an independent assortment of genes into gametes. The correct option is a).
Step-by-step explanation:
The cellular process that underlies Mendel's law of independent assortment is the random alignment of chromosomes during metaphase I of meiosis I. This random orientation of tetrads ensures that each gamete can contain any possible combination of paternal and maternal chromosomes.
Subsequently, during anaphase I, the homologous chromosomes separate, finalizing the assorted combinations of chromosomes for each gamete at the end of meiosis.
This principle provides genetic diversity and explains why different genes assort independently during gamete formation unless they are closely linked on the same chromosome, in which case recombination may further increase genetic variation. Option a) is the correct one.