During the early stages of meiosis, two chromosomes in a homologous pair may exchange segments, producing genetic variation in sex cells. this process of exchange is called a) transcription. b) crossing over. c) nondisjunction. d) genetic shuffling.

The correct answer is option B, that is, crossing over.  

Crossing over or chromosomal crossover refers to the interchange of genetic substance between the two homologous chromosomes non-sister chromatids, which leads to the formation of recombinant chromosomes at the time of sexual reproduction.  

It takes place in the pachytene stage of prophase I of meiosis at the time of a procedure known as synapsis.