Step 1: Understanding the Concept
Ascorbic acid is vitamin C. Its deficiency causes scurvy.
Step 2: Detailed Explanation
Vitamin C is plentiful in citrus fruits such as oranges and lemons, amla and guava. Cinnamon, turmeric and clove are spices and are not major sources.
Final Answer:
Citrus fruits are a natural source, option (A).
\[ \boxed{\text{Citrus fruits}} \]