Final answer:
The school nurse's drug awareness program would most likely focus on 15- to 17-year-olds, as adolescence is associated with a higher risk of experimental behavior and drug use (d).
Step-by-step explanation:
The school nurse is focusing a drug awareness program on the age group with the largest increase in drug use. As the question does not provide specific statistical data on age groups and their recent changes in drug use trends, we can use general knowledge and context from various drug awareness and education sources to infer the most likely answer. Typically, the period of adolescence is associated with a higher likelihood of experimental behavior and exposure to substance use.
Given this understanding and considering cognitive development and social factors, option d. 15- to 17-year-olds is commonly recognized as an age group with a significant increase in drug use, especially as they approach the later stages of high school and are exposed to a wider range of social circles and pressures.