Introduction:
The question asks about the source of stains used for lowering blood cholesterol levels. Cholesterol is a type of fat found in the bloodstream and is essential for various bodily functions. However, high levels of cholesterol can lead to health problems, particularly cardiovascular diseases. Thus, it is important to manage cholesterol levels. Stains are commonly prescribed medications that help in reducing cholesterol levels.

Explanation:
The correct answer to the question is option 'D' - yeast. Stains used for lowering blood cholesterol levels are extracted from yeast. Here's an explanation of why yeast is used and how it helps in reducing cholesterol:

1. Yeast and Lovastatin:
- Yeast is a type of fungus that is commonly used in the production of various food and beverage products like bread, beer, and wine.
- Certain species of yeast, particularly Monascus purpureus, produce a compound called lovastatin.
- Lovastatin is a type of statin, which is a class of medications commonly prescribed as cholesterol-lowering drugs.
- Statins work by inhibiting an enzyme called HMG-CoA reductase, which is involved in the production of cholesterol in the body.
- Lovastatin specifically inhibits this enzyme, leading to a decrease in the production of cholesterol.

2. Production of Lovastatin:
- Lovastatin is produced by fermenting Monascus purpureus with a specific strain of yeast, Aspergillus terreus.
- During the fermentation process, Aspergillus terreus converts certain compounds present in Monascus purpureus into lovastatin.
- The resulting lovastatin is then extracted and purified to be used as a medication for lowering blood cholesterol levels.

3. Effectiveness and Safety:
- Lovastatin and other stains derived from yeast have been extensively studied and proven to be effective in reducing cholesterol levels.
- They are commonly prescribed by healthcare professionals for individuals with high cholesterol levels or those at risk of cardiovascular diseases.
- However, like any medication, stains may have potential side effects and should be used under medical supervision.

Conclusion:
Stains used for lowering blood cholesterol levels are extracted from yeast. Yeast, specifically Monascus purpureus, produces a compound called lovastatin, which is a type of statin medication. Lovastatin inhibits the enzyme involved in cholesterol production, thereby helping to reduce cholesterol levels in the bloodstream. It is important to note that stains should be used under medical supervision, as they may have potential side effects.

Answer is b