Sexual reproduction is absent in

The correct option is Nostoc.

Step-by-Step Explanation:

1. Understanding Nostoc:
Nostoc is a genus of cyanobacteria (blue-green algae) which belongs to the kingdom Monera (prokaryotes). Prokaryotic organisms do not possess a true nucleus or membrane-bound organelles, and they lack the mechanisms for meiosis and gamete formation. Consequently, true sexual reproduction is completely absent in Nostoc. Instead, they reproduce vegetatively or asexually through methods like fragmentation, hormogonia formation, or akinetes.

2. Analyzing the other options:
- Volvox: A genus of chlorophyte green algae. It exhibits oogamous sexual reproduction, where a small, motile male gamete fertilizes a large, non-motile female gamete.
- Aspergillus: A genus of fungi (Ascomycetes) that can undergo sexual reproduction through the formation of cleistothecia containing ascospores, in addition to reproducing asexually.
- Ulothrix: A genus of filamentous green algae. It undergoes sexual reproduction, typically of the isogamous type, where two flagellated gametes of similar size fuse.

3. Conclusion:
Therefore, among the given choices, sexual reproduction is absent only in Nostoc.