Solution:
Nostoc belongs to cyanophyceae. In this entire class sexual reproduction is completely absent. It reproduces by colony formation, hormogonia, akinetes, heterocysts and endospres. However genetic recombination has been observed. It may be probably through transformation or conjugation. The other three algae- Spirogyra, Ulothrix and Volvox belong to chlorophyceae. The members of this class show isogamous, anisogamous and oogamous type of sexual reproduction.
