Six nutrients needed by human body: protein, sugar, oil, vitamins, inorganic salts and water; Combined with the meaning of the question, the main component of rice is sugar, tofu and braised pork contain oil, protein and inorganic salts, indicating that the vitamin content in food is less; Therefore, in order to make the nutritional collocation more reasonable, it is necessary to supplement foods rich in vitamins.

A, cold cucumber contains vitamins, so the option is correct;

B, fried peanuts are rich in protein and oil, so the option is wrong;

C, fried eggs are rich in protein and oil, so the option is wrong;

D Carassius auratus soup is rich in protein and oil, and the option is wrong.

So choose a.