Tetrad is the structure formed during the meiotic type of cell division which produces four daughter cells.
The tetrad structure is seen under the microscope during the pachytene substage of prophase I.
The tetrad structure is formed when four sister chromatids are aligned together in a cell before the crossing over event or recombination has to take place. The tetrad formation allows the crossing over to take place in pachytene.
Thus, pachytene, prophase I is the correct answer.
