Choose the one word substitution for the following description: No longer a child, but not yet an adult.

Introduction / Context:
This question focuses on age related terminology. It is important in sociology, psychology, and everyday English to describe different stages of human development accurately. The given description refers to the transitional phase between childhood and adulthood.

Given Data / Assumptions:
- Description: No longer a child, but not yet an adult. - Options: Youngster, Adolescent, Juvenile, Yokel. - The word must denote a specific life stage rather than just youth or immaturity in general.

Concept / Approach:
Adolescent is the term used in psychology and common language to refer to a person in the teenage years, between childhood and adulthood. The phrase adolescent years usually covers roughly ages thirteen to nineteen. Other options either have broader or different meanings, or refer to social types rather than age groups.

Step-by-Step Solution:
Step 1: Identify that the description points exactly to the teenage transition stage. Step 2: Recall that adolescent describes someone in this transitional age between child and adult. Step 3: Examine Youngster. This is a general term for a young person and can include both children and adolescents. Step 4: Examine Juvenile. This can mean young, but is often used in legal contexts for minors or in a negative way for childish behaviour. Step 5: Examine Yokel, which is an informal word for an unsophisticated person from the countryside and has nothing to do with age stage. Step 6: Therefore, Adolescent best matches the definition.

Verification / Alternative check:
Look at example usage: Adolescent psychology focuses on issues experienced during the teenage years. A person of this age is called an adolescent. In contrast, juvenile court deals with all minors, including younger children, and youngster is a loose word for any young person. The definition no longer a child, but not yet an adult matches adolescent most closely in both ordinary and technical usage.

Why Other Options Are Wrong:
Youngster is too broad and does not clearly specify the transitional stage between child and adult. Juvenile is often used in legal or negative contexts and again covers all minors, not specifically the transition age. Yokel describes a naive or unsophisticated rural person, with no reference to age category.

Common Pitfalls:
Many learners select juvenile because they associate it with youth crimes and teenage offenders. However, the question is neutral and about developmental stage, not legal status. Remember that adolescent has a more precise psychological and biological meaning, which fits the description of moving from childhood toward adulthood.

Final Answer:
The correct one word substitution is Adolescent.