B. A food that makes up a significant portion of a person's diet This definition best fits the concept of a staple food.

Part II
D. Bicycles must not be left in the hall. This is the most grammatically correct and concise way to express the prohibition.
B. The customer wanted to know what time the manager comes back. This sentence accurately conveys the customer's question in reported speech.
D. If you don't work much harder, you'll fail the exam. This sentence expresses the conditional relationship between effort and outcome.
B. In spite of the bad weather, the farmers kept on working in the fields. This sentence correctly uses the phrase "in spite of" to show contrast.
D. The old man wishes his grandchild visited him more often. This sentence correctly expresses the old man's desire.
D. Linda used to be my best friend. This sentence correctly uses the past tense to indicate a past state.
B. The old man whose bicycle I borrowed yesterday is working in this factory. This sentence correctly uses the relative clause to identify the old man.
C. It is impossible for a fish to climb a tree. This sentence is the most natural and grammatically correct way to express the impossibility.