Crossing over refers to

The correct option is "exchange of segments between sections of chromosome".

Step-by-step Explanation:

1. Understanding Chromosomes and Genetics:
Chromosomes are long thread-like structures made of DNA and proteins that carry genetic information in the form of genes. In diploid organisms, chromosomes exist in homologous pairs, where one chromosome is inherited from the mother and the other from the father.

2. What is Crossing Over?
Crossing over, also known as homologous recombination, is a fundamental biological process that occurs during Prophase I of meiosis (specifically during the pachytene stage). During this process, non-sister chromatids of homologous chromosomes pair up and align closely with each other.

3. The Mechanism:
When these homologous sections align, physical breakage and reciprocal reconnection occur between non-sister chromatids. This leads to a physical exchange of genetic material (DNA segments) between the maternal and paternal chromosomes. The points where this physical exchange occurs are called chiasmata.

4. Why the Other Options are Incorrect:
- Thickening of chromosome: This is a physical condensation process that occurs during prophase to make chromosomes visible, but it does not involve the exchange of genetic material.
- Change in chromosome / Mutation: While crossing over creates new combinations of alleles (recombination), it is a normal, regulated process of genetic exchange rather than a random mutation or abnormal change in chromosome structure.
Therefore, crossing over refers precisely to the reciprocal exchange of segments between sections of homologous chromosomes, which generates genetic diversity in offspring.