"Specificity and progression" are two principles related to maintaining physical health. Therefore we can say that the word "principles" is the most suitable to replace the word "thing".
Also, "Specificity and progression" are neither exercises nor muscles. Nor are they acronyms, since an acronym is the word formed by joining letters or initial syllables of a group of words, pronounced as one word.
Acceptable answers could be A because if it's on the side people will feel less tempted to eat it and will eat less. C techinally could be correct but i don't think it's the answer.
