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

Correct
"B- homologous chromosomes join together to form tetrads during prophase I"

It is during prophase I that homologous chromosomes join together (synapsis) and form tetrads - four chromatids are together in the new structure of two chromosomes - and this is the reason why crossing-over occurs in this phase. It is in this tetrad that both arms of both chromosomes may crossover and matching regions exchange places. This process results in homologous chromosomes recombination leading to genetic variability. 

Ithink that b is correct but just in case wait to see if someone else answers