Step 1: Definition under Factories Act.
The Act defines:
- Child: below 15 years.
- Adolescent: more than 15 years but less than 18 years.
Step 2: Elimination of wrong options.
- (A) Completing 17 years does not alone define adolescent.
- (B) Less than 18 years includes even children.
- (D) Incorrect as (C) is correct.
Step 3: Conclusion.
Correct definition is (C).