The correct answer is A. Support
Explanation:
In language, the meaning of a word is often modified or suggested by the context of it. In the case of the word "sustain", this is used to explain the way a variety of activities contribute to a long-term fitness routine in "Choosing a variety of activities that you enjoy can help sustain a long-term fitness routine". This implies, in this case, sustain means to keep up, maintain or support. Thus, in this sentence, the word sustain means support rather than prevent, create or explain.
