Which of the following processes occurs when homologous chromosomes cross over in meiosis I?

a) Two sister chromatids get tangled, resulting in one re-sequencing its DNA.

b) Two sister chromatids exchange identical pieces of DNA.

c) Maternal alleles are "corrected" to be like paternal alleles and vice versa.

d) Corresponding segments of non-sister chromatids from homologous chromosomes are exchanged.