(c) scalariform conjugation

Explanation:

Conjugation is exchange of genetic material i.e. mode of sexual reproduction in lower organisms.

Conjugation occurs by two methods:

(i) Scalariform conjugation: It takes place during the night and between the recently divided cells of the conjugating participating filaments. Opposite cells of two filaments develop conjugation tube by lying side by side and parallel to each other. The conjugation tube between two filament looks like a ladder, through which gamete from one gametangia passes through to fuse with the passive gamete of another filament.

(ii) Lateral conjugation: Two adjacent cells of same filament function as male and female cells. It is commonly reported in S. affinis and S. fenuissima.