The sexual reproduction is absent in

Explanation

b. Nostoc belongs to Cyanophyceae. In this entire class sexual reproduction is completely absent. It reproduces by colony formation, hormogonia, akinetes, heterocysts, and endospores. However genetic recombination has been observed. It may be probably through transformation or conjugation. The other three algae­ Spirogyra, Ulothrix andVolvox belong to Chlorophyceae. The members of this class show isogamous, anisogamous, and oogamous types of sexual reproduction. Explain other options too: a. Spirogyra: Spirogyra reproduces both sexually and asexually. It undergoes a process called conjugation for sexual reproduction, where two filaments come into contact, and a conjugation tube forms between them. However, it doesn't involve the formation of specialized reproductive structures like gametes. c. Ulothrix: Ulothrix is a filamentous green alga. It reproduces both sexually and asexually. Asexual reproduction involves the formation of zoospores, and sexual reproduction involves the formation of isogametes (similar-sized gametes). d. Volvox: Volvox is a colonial green alga. It also reproduces both sexually and asexually. Asexual reproduction occurs through the formation of daughter colonies, while sexual reproduction involves the formation of specialized reproductive cells, including eggs and sperm.