When homologous chromosomes crossover, what occurs?
a) two chromatids get tangled, resulting in one re-sequencing its dna.
b) two sister chromatids exchange identical pieces of dna.
c) specific proteins break the two strands and re-join them with their homologs.
d) each of the four dna strands of a tetrad is broken and the pieces are mixed.
e) maternal alleles are "corrected" to be like paternal alleles and vice versa.

B) Two sister chromatids exchange identical pieces of DNA. 

The homologous chromosomes have the same genes in common. Each cell has at least two sets of chromosomes which one comes from one’s father also called as paternal chromosome and mother, called as the maternal chromosomes. These chromosomes are what makes the child obtain the characters and may depend on the traits the parents possess. Homologous chromosomes are not duplicated chromosomes or identical however, they are analogous or similar.