Step 1: Understanding the Problem:
The question asks to identify the predominant organic acid found in grapes (Vitis vinifera), which contributes to their characteristic tartness and play an important role in winemaking.
Step 2: Detailed Explanation:
• Organic Acids in Fruits: Organic acids are critical to the sensory profile, preservation, and pH of fruits. Different fruits have distinct acidic profiles.
• Tartaric Acid in Grapes: Grapes are unique because tartaric acid is their predominant organic acid, accounting for up to $50\% - 60\%$ of the total acid content. Tartaric acid is relatively rare in other common fruits.
• Malic Acid: This is the second most abundant acid in grapes, but it decreases during ripening as it is metabolized by the plant. It is the predominant acid in apples and stone fruits.
• Significance in Winemaking: Tartaric acid is highly stable and resistant to microbial degradation. It plays a key role in maintaining the low pH of wine, which is critical for color stability and biological preservation. Its salts (potassium bitartrate) can precipitate to form "wine diamonds."
• Citric Acid: The predominant acid in citrus fruits (lemons, oranges), present only in trace amounts in grapes.
Step 3: Final Answer:
Tartaric acid is the predominant organic acid in grapes, making option (C) the correct choice.