Biotin, also known as Vitamin B7, is a water-soluble vitamin that plays a vital role in metabolism. While it is found in various foods, Cauliflower is recognized as a significant plant-based source of this nutrient.
Step 1: Analyze the options.
(A) Potato: While potatoes contain various vitamins, they are not a primary or notable source of Biotin.
(B) Watermelon: This fruit is mostly water and contains Vitamin C and A, but negligible amounts of Biotin.
(C) Cauliflower: One of the best vegetable sources of Biotin; consuming it raw or lightly cooked preserves the vitamin content.
(D) All of these: Since options (A) and (B) are not significant sources, this option is incorrect.
Step 2: Conclusion.
Among the given choices, Cauliflower is the established source of Biotin. Hence, the correct answer is (C).
Final Answer:
\[
\boxed{(C) \, \text{Cauliflower}}
\]