Question

how can teens model good physical fitness to other teens in their neighborhood?
by reading books at the library
by baking cookies for the elderly
by riding their bikes to school
by growing vegetables in their own gardens

Explanation:

To model good physical fitness, an activity that involves physical exercise is needed. Reading books (A) is a sedentary activity. Baking cookies (B) and growing vegetables (D) are also not physical - fitness - related activities. Riding bikes to school (C) is a form of physical exercise, so it is the way teens can model good physical fitness to other teens.

Answer:

C. by riding their bikes to school