The correct answer is (c). Chiasmata are formed during prophase I of meiosis. They are the physical manifestation of the crossing over that occurs between homologous chromosomes during this stage. Crossing over is the exchange of genetic material between homologous chromosomes, and it is a key event in meiosis that increases genetic diversity.
During prophase I, the chromosomes condense and become visible. The homologous chromosomes pair up and form bivalents. The synaptonemal complex forms between the homologous chromosomes, and this is where crossing over occurs. The chiasmata are visible as the points where the synaptonemal complex breaks and reforms.
After crossing over, the homologous chromosomes are still held together by the chiasmata. This allows them to be pulled apart during anaphase I. The chiasmata eventually disappear during telophase I.
(a) Metaphase I is the stage of meiosis where the chromosomes line up at the metaphase plate. The homologous chromosomes are still paired up at this stage, but the chiasmata are not visible.
(b) Anaphase I is the stage of meiosis where the homologous chromosomes are pulled apart to opposite poles of the cell. The chiasmata are still visible at this stage, but they are not necessary for the separation of the homologous chromosomes.
(d) Telophase I is the stage of meiosis where the chromosomes decondense and the nuclear envelope reforms. The chiasmata have disappeared by this stage.