Final answer:
Girls generally achieve complete physical maturity by 19 years old, which includes the end of growth in height and full development of secondary sexual characteristics.
Step-by-step explanation:
Girls are usually completely physically mature by 19 years old. Puberty in girls can start between the ages of nine and ten, with the development of the breasts and pubic hair. The process of physical maturation continues through adolescence, with the adolescent growth spurt occurring and significant changes in body composition and shape. Moreover, the reproductive organs such as the uterus and vagina develop after the onset of breast development, with menarche usually occurring around 12-13 years old. While significant changes occur as early as 14 to 16 years old, physical maturity is typically reached in the late teenage years or early twenties.
Based on the information provided, adulthood begins around 20 years old, which is when physical maturation is complete. This means that by the age of 19, most girls would have completed their growth in height and development of secondary sexual characteristics, such as breast growth and the onset of menstruation. Therefore, the best answer from the choices provided is B. 19 years old.