Girls are usually completely physically mature by the time they are years old.
a. 15 to 17
b. 17 to 19
c. 12 to 15
d. 18 to 21

The question is asking us to say when girls are completely physically mature. Some girls go through puberty as easly as at 10 years of age, but most start at 12. At the very latest, some girls go through puberty just before they're 18. The best answer is 15 to 17 - if you choosse an earlier option, then the "late bloomers" will be excluded, but also many girls mature before they're 17, soo the other options would leave to many girls out