Fertilisation is internal in spirogyra. Where would you get zygospores, if 3 filaments are conjugating by scalariform conjugation?

In Spirogyra, both lateral and scalariform conjugation has been observed. During the scalariform conjugation, a pair of filaments, either central or lateral, align themselves laterally and connects through branch-like conjugation tubes. This tube grows from each of the filament and meets in the centre to form a space in which the zygote moves. Each zygote will grow in a filament. During the lateral conjugation, gametes produced by adjacent cells on the same filament meet and develop within a short, curved conjugation tube. So, the correct option is D.