To solve this problem, we need to identify which organism among the given options is not a prokaryote.$\newline$Prokaryotes are organisms that lack a true nucleus and membrane-bound organelles. They include bacteria and archaea.$\newline$Let's analyze each option:$\newline$• Option 1: $$Nostoc$$ is a genus of cyanobacteria, which are prokaryotic organisms.$\newline$• Option 2: $$Mycobacterium$$ is a genus of bacteria, which are prokaryotic organisms.$\newline$• Option 3: $$Saccharomyces$$ is a genus of fungi, specifically yeast, which are eukaryotic organisms.$\newline$• Option 4: $$Oscillatoria$$ is a genus of cyanobacteria, which are prokaryotic organisms.$\newline$From the analysis, we can see that Saccharomyces is the only eukaryotic organism among the options.$\newline$Therefore, the correct answer is Option 3: Saccharomyces.