Concept:The word "deleterious" means something that causes harm or damage to a person or thing.
Explanation:"Deleterious" is an adjective used to describe an effect that is injurious, damaging, or destructive.
In the sentence, the drugs are said to have a deleterious effect on a child's development, meaning the drugs are doing something bad to the child's growth.
Now let us examine each option carefully.
Option A, "debilitating," means weakening or reducing strength; this is close in idea but not the most direct match.
Option B, "helpful," means useful or beneficial, which is the opposite of the underlined word.
Option C, "harmful," means causing damage, injury, or negative consequences; this is exactly what "deleterious" conveys.
Option D, "healing," means curing or restoring to good health, which also has a positive meaning and is not correct.
Therefore, the option that is nearest in meaning to "deleterious" is "harmful."
Answer:Option C. harmful