To solve this problem, we need to identify which organism completely lacks a cell wall. Let's analyze each option:$\newline \bullet$Option 1: Cyanobacteria$\newline$ - Cyanobacteria, also known as blue-green algae, are a group of photosynthetic bacteria.$\newline$ - They possess a cell wall made of peptidoglycan, similar to other bacteria.$\newline$ - Therefore, cyanobacteria do not completely lack a cell wall.$\newline \bullet$Option 2: Sea-fan (Gorgonia)$\newline$ - Sea-fans are marine organisms belonging to the phylum Cnidaria.$\newline$ - They are multicellular animals and, like all animal cells, they lack a cell wall.$\newline$ - Therefore, sea-fans completely lack a cell wall.$\newline \bullet$Option 3: Saccharomyces$\newline$ - Saccharomyces is a genus of fungi, commonly known as yeast.$\newline$ - Fungi have a cell wall composed of chitin.$\newline$ - Therefore, Saccharomyces does not completely lack a cell wall.$\newline \bullet$Option 4: Blue-green algae$\newline$ - Blue-green algae is another name for cyanobacteria.$\newline$ - As mentioned earlier, they have a cell wall made of peptidoglycan.$\newline$ - Therefore, blue-green algae do not completely lack a cell wall.$\newline$Based on the analysis, the organism that completely lacks a cell wall is:$\newline$Option 2: Sea-fan (Gorgonia).$\newline$Thus, the correct answer is Option 2.