Vitamins are essential organic compounds that our bodies need in small quantities for various physiological functions. They cannot be synthesized in sufficient amounts by our bodies, so we must obtain them through our diet or supplements. The correct answer to the given question is option 'A', which states that vitamins are required in small quantities to prevent deficiency diseases.

Deficiency Diseases:
- Vitamins play a crucial role in preventing deficiency diseases. These diseases occur when our bodies do not receive an adequate amount of specific vitamins. Deficiency diseases can lead to various health problems and impair the normal functioning of our bodies. For example:
- Lack of vitamin C can cause scurvy, which results in fatigue, bleeding gums, and weakened immune function.
- Insufficient vitamin D can lead to rickets, a condition characterized by weak and deformed bones.
- A deficiency of vitamin A can cause night blindness and dry skin.

Vitamin Functions:
- Each vitamin has its specific function in the body. Some key functions of vitamins include:
- Vitamin A: Supports vision, immune function, and cell growth.
- Vitamin B complex: Plays a crucial role in energy production, brain function, and the formation of red blood cells.
- Vitamin C: Essential for collagen production, wound healing, and immune system support.
- Vitamin D: Promotes calcium absorption, bone health, and immune function.
- Vitamin E: Acts as an antioxidant, protecting cells from damage, and supports immune function.
- Vitamin K: Important for blood clotting and bone health.

Preventing Deficiency Diseases:
- By consuming a balanced diet that includes a variety of fruits, vegetables, whole grains, lean proteins, and dairy products, we can obtain an adequate amount of vitamins to prevent deficiency diseases.
- In some cases, individuals may require vitamin supplements to meet their nutritional needs, especially if they have specific dietary restrictions or medical conditions that affect nutrient absorption.

Conclusion:
Vitamins are substances that are required in small quantities to prevent deficiency diseases. They play essential roles in various bodily functions and are necessary for maintaining overall health and well-being. A balanced diet and, if necessary, supplements can help ensure an adequate intake of vitamins to prevent deficiency diseases.