Name the event wherein the paternal and maternal chromosomes change their material with each other in cell division

The correct option is Crossing over.

During cell division, specifically in meiosis I (prophase I), homologous chromosomes pair up. In this paired state, non-sister chromatids of maternal and paternal origin can physically overlap and exchange segments of genetic material. This reciprocal exchange of homologous segments between non-sister chromatids of a bivalent is known as crossing over.

Let's look at why the other options are incorrect in this context:

1. Synapsis: This refers to the pairing of homologous maternal and paternal chromosomes, which occurs before crossing over. It does not involve the exchange of genetic material itself.
2. Bivalent forming: A bivalent is the pair of homologous chromosomes formed during synapsis. This is a structural state, not the process of material exchange.
3. Dyad forming: A dyad refers to a pair of sister chromatids joined at the centromere, which become visible later in meiosis. It is not the exchange event.

Therefore, the process where paternal and maternal chromosomes exchange genetic material is called Crossing over.