Tetras form during
a. prophase i b. metaphase i c. prophase. ii d. metaphase ii

Tetrads are formed during Prophase I of meiosis.

Explanation:

Meiosis is the cell division in which a single interphase is followed by two consecutive nuclear division.This results in four haploid daughter cells from a single diploid mother cell.The two nuclear division are termed as -Meiosis I and Meiosis IIMeiosis I is the reductional division and Meiosis II is the Equational division.Meiosis I consists of :prophase I, Metaphase I, Anaphase I Telophase I.Again the prophase I is divided into five subphases like:Leptotene,Zygotene, Pachytene, Diplotene diakinesis.During the Zygotene phase, two homologous chromosomes, one being derived from the female and the other from the male parent remain  paired with each other throughout the chromosome length and form a synaptonemal complex.These paired homologous chromosomes  are called tetrad which are clearly visible in pachytene.

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