Final answer:
The term 'adolescence' became widely used in the 20th century, specifically during the Age of Adolescence (1890-1920).
Step-by-step explanation:
The term adolescence became widely used in the 20th century, specifically alluding to the time period known as the Age of Adolescence (1890-1920). This era saw a heightened focus on the developmental stage that bridges childhood and adulthood due to industrialization, which required a more educated workforce and delayed entry into the labor market. The correct answer to when the term adolescence become a widely used term is d) In the 20th century.