Step 1: Recall common dietary sources of Vitamin C.
Vitamin C (ascorbic acid) is abundantly found in citrus fruits.
Step 2: Analyse each option.
Rice: Mainly a source of carbohydrates, negligible Vitamin C.
Milk: Contains proteins and calcium, very little Vitamin C.
Egg: Rich in proteins and fats, not a significant source of Vitamin C.
Lemon: A citrus fruit, rich in Vitamin C.
Step 3: Select the correct option.
Among the given options, lemon is the richest source of Vitamin C.
Hence, the correct answer is
\[
\boxed{\text{Lemon}}
\]