Six nutrients needed by human body: protein, sugar, oil, vitamins, inorganic salts and water; Combined with the meaning of the question, rice and bread are rich in starch and belong to sugar; Steamed pumpkin, potato chips and vinegar-mixed cucumber contain vitamins, oils and inorganic salts, indicating that the content of protein in food is less; Therefore, in order to make the nutrition collocation more reasonable, it is necessary to supplement foods rich in protein.

A. Liang Xiao's lack of nutrition in dinner recipes is protein, not sugar, so this choice is wrong.

B, the nutrient missing in Xiao Liang's dinner recipe is protein, so the choice is correct.

C. Liang Xiao's dinner menu lacks nutrition of protein, not oil, so this choice is wrong.

Xiao Liang's dinner recipe lacks protein, not vitamins, so the choice is wrong.

Therefore, choose: B.