The answer is C. Targeting a particular area for improvement
Specificity principle stated that whatever you do in the gym, should be relevant to your desired outcome
For example, if you are a rowing athlete, it should be best if you focus on upper body training more than lower body because you need your upper body strength more than you need the lower
You can prevent the spreading of communicable disease by following through with Option C) avoiding sharing personal items, like toothbrushes. All of the other options can lead you to getting a disease, if you ignore hygiene you will never be clean, and if you avoid insect repellent you might get a disease from a bug, and if you cough/sneeze without covering your nose/mouth then you may catch a cold. Therefore pointing to the correct answer option C.
I hope this helps! Have a good day!