During the prophase-I, the crossing over is occurs at the time of meiosis.
Meiosis division in a reproductive cell often results in four haploid cells with only single set of chromosomes.
Meiosis-I prophase-I In this process double stranded homologous chromosomes line up each other, and result in forming a tetrad; this is termed as Synapsis.
Terminal chaismata hold the homologous chromosomes together in metaphase I finally these are going to align on the metaphase plate. Synapsis allows homologous chromosomes to travel to the center by forming spindle formation through microtubules attaching to the centromeres finally to the opposite poles of the cell. Later, crossing over takes place to exchanges paternal and maternal genes to form tetrad where genetic variation occurs. Crossing over takes place to exchange equal amounts of DNA generates genetic variability in prophase -I of meiosis where there is no change in DNA structure and DNA is double stranded even at crossing over formation of reduction division.
Crossing over occurs in which lined up chromosomal ends exchange genetic information by switching their places. Crossing over can unlink the genes.
Thus, the options (d), (c) and (b) are incorrect.
Meiosis-I prophase-I In this process double stranded homologous chromosomes line up each other, and result in forming a tetrad; this is termed as Synapsis.
Therefore, the correct option is – A. Prophase I.