What event is important factor in increasing genetic diversity among sexually reproducing organism

The correct answer would be crossing over.

Crossing over is an important event in which exchange of genetic material between non-sister chromatids of homologous chromosomes takes place.

It occurs during prophase 1 (particularly in pachytene sub-stage) of the meiosis 1.

During prophase 1, the pair of homologous chromosome are aligned in the process termed as synapsis after which they exchange the genetic material at the points of chiasmata.

It results in formation of new combination of alleles in chromosomes which leads to the development of genetic diversity in sexually reproducing organisms.

correct answer is a. and d.