Primary prevention is an active step taken to head off or mitigate a disease- think of vaccinations. In this case, it is lifestyle modification (exercise) to help 30-40 year old women (weight bearing exercise can mitigate osteoporosis, benefits of aerobic exercise).
Seconday prevention is early detection of a disease, promoting early intervention. This could inclue things like HIV testing every 6 months, or or annual mammography.
Tertiary intervention is therapy aiming at reducing complications, allowing relapse, or improving function. Abstinence from alcohol may be here because of substance abuse problems, or because some damage previously done by alcohol cannot be prevented. Similarly for a low carbohydrate diet we may be mitigating the effects on metabolism that have already been done.
Wasn't sure about others, but mammography for general population isn't recommended until 40
Primary to Prevent - Vaccine, Exercise, Not eating sugar to prevent cavities
Secondary to Screen - X-rays to check tooth decay