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

The correct option is Penicillium notatum.

Roquefort cheese is a world-famous variety of blue cheese known for its distinct sharp flavor and blue-green veins. These characteristic features are produced by ripening the cheese with a specific species of mold (fungus) belonging to the genus Penicillium. Traditionally, the specific species used is Penicillium roqueforti. Among the options provided, Penicillium notatum is the representative member of this fungal genus.

To understand why this is the correct choice, we can evaluate the roles of the other listed microorganisms:
• Saccharomyces cerevisiae: Commonly known as baker's yeast or brewer's yeast, this unicellular fungus is primarily used for bread-making and alcohol fermentation, not for cheese ripening.
• Propionibacterium sharmanii: This is a species of bacterium (not a fungus) responsible for the fermentation of Swiss cheese, producing large carbon dioxide (CO₂) bubbles that form the characteristic holes (eyes) in the cheese.
• Monascus purpureus: This fungus is used in the production of red yeast rice and is commercially important for producing statins, which are used as blood-cholesterol-lowering agents.

Since the ripening of Roquefort cheese is uniquely driven by a species of the genus Penicillium, Penicillium notatum is the correct option among the choices provided.