crossing over occurs when

a sister chromatids are pulled apart at the centromere during anaphase ii.

b homologous chromosomes join together to form tetrads during prophase i.

c chromosomes are grouped together into new nuclei during telophase ii.

d tetrads of homologous chromosomes randomly line up in metaphase i.

The correct answer is option B (homologous chromosomes join together to form tetrads during prophase I).

Explanation:

Crossing over is a mechanism in which the genetic material is exchanged between 2 non-sister chromatids of homologous chromosomes.

It takes place by a process called synapsis which takes place during pachytene substage of prophase I during meiosis I when two homologous chromosomes, one from each parent pair with each other with the help of a protein-RNA structure called synaptonemal complex which forms tetrad.

During this, DNA sequences are exchanged between the homologous chromosomes producing recombinant chromosomes.

Thus, option B is the correct answer.