Step 1: Understanding contextual usage.
In the sentence, "junk food" refers to food that is unhealthy and lacks nutritional value. It is not garbage or rusty but something children eat that is not nutritious.
Step 2: Evaluating the options.
(A) not providing nutrition: This matches the meaning of junk food.
(B) garbage: Too literal; junk food is not actual garbage.
(C) unusable: Incorrect meaning.
(D) rusty: Not relevant to food.
Step 3: Conclusion.
Therefore, the correct contextual meaning of "junk" here is "not providing nutrition."