Identify the fungus that ripens the famous 'roquefort' cheese :

The correct option is Penicillium notatum.

Step-by-Step Explanation:
To identify the correct organism, let's analyze the role of each option provided:
1. Saccharomyces cerevisiae: Commonly known as Baker's or Brewer's yeast, this fungus is widely used in baking bread and fermenting beverages like beer and wine, not for cheese ripening.
2. Propionibacterium sharmanii: This is a bacterium (not a fungus) responsible for the ripening and large hole formation in Swiss cheese through the production of carbon dioxide gas.
3. Monascus purpureus: This yeast is used in the production of red yeast rice and is famous for producing statins, which are blood cholesterol-lowering agents.
4. Penicillium notatum: This belongs to the genus Penicillium. While the famous Roquefort cheese is specifically ripened by the closely related species Penicillium roqueforti to give it its characteristic blue veins and sharp flavor, Penicillium notatum (renowned as the source of the first antibiotic, penicillin) is the only representative of the cheese-ripening Penicillium genus among the choices.

Therefore, based on the options provided, Penicillium notatum is the correct choice.