Step 1: Understanding the Concept:
Ascorbic acid is the chemical name for vitamin C, a water-soluble vitamin.
Step 2: Detailed Explanation:
Vitamin C is found abundantly in citrus fruits such as oranges, lemons and amla, and also in guava and tomato. Deficiency leads to scurvy, with bleeding gums and slow wound healing.
Step 3: Why the other options are wrong.
Clove (A) and cinnamon (D) are spices known for essential oils, such as eugenol and cinnamaldehyde, not for vitamin C. Wintergreen (B) is the source of methyl salicylate, used as a pain reliever.
Final Answer:
Citrus fruits are a natural source of ascorbic acid, option (C).
\[ \boxed{\text{Citrus fruits (C)}} \]