Deficiency … just one essential vitamin may cause illness.

Analyze the question: The sentence is "Deficiency ____ just one essential vitamin may cause illness." We need the correct preposition to follow the noun deficiency.

Explain the concept: In English, some nouns are followed by specific prepositions. This is called a dependent preposition. The noun deficiency takes in when we name what is lacking:

• a deficiency in iron
• a deficiency in vitamin C
• a deficiency in protein

Check each option:

• A. on: "deficiency on" is not idiomatic.
• B. in: "deficiency in" is the standard collocation. ✔
• C. from: we say "suffer from a deficiency", but the noun itself is not followed by from.
• D. for: "deficiency for" is not used to name the missing substance.

Solution: The correct sentence is: "Deficiency in just one essential vitamin may cause illness."

Tip: Learn noun and preposition pairs as fixed chunks. For example, deficiency in, lack of, shortage of, excess of, increase in, demand for. When you see "deficiency," think "deficiency in." Note that "suffer from a deficiency" is a different structure, because there the preposition follows the verb suffer.