Tetrad formation occur in sub-stage:

Explanation

Substages of meiosis where tetrad formation occurs:Leptotene: During the leptotene substage, chromosomes condense and become visible under a microscope. However, tetrad formation has not yet occurred at this stage.Zygotene: Tetrad formation begins during the zygotene substage. Here, homologous chromosomes start to pair up, forming structures called tetrads. This process is also known as synapsis.Pachytene: Tetrad formation continues into the pachytene substage.In pachytene, homologous chromosomes are fully aligned and tightly connected, facilitating genetic recombination through crossing over.Diplotene: During the diplotene substage, tetrads begin to separate, but chiasmata (crossing over points) are still visible, indicating that genetic exchange has occurred.Diakinesis: In diakinesis, the final substage of meiosis I, tetrads further condense, and the chiasmata move to the ends of the chromosomes. This stage marks the completion of tetrad formation and sets the stage for chromosome segregation during meiosis.Thus, correct answer is Zygotene as tetrad formation occurs here.