Answer:
(D) Strenuous.
Step-by-step explanation:
The word that can be substituted for "vigorous" without changing the meaning is "strenuous".
Both "vigorous" and "strenuous" are adjectives that describe something that requires great physical effort or energy. In the context of the question, Erin was following a vigorous exercise program, which means the program was intense and required a lot of effort. If we replace "vigorous" with "strenuous", the meaning remains the same.
For example, if Erin was following a strenuous exercise program, it would still imply that the program was demanding and required a high level of physical exertion.
The other answer choices, "sour", "determined", and "physically", do not have the same meaning as "vigorous". "Sour" means having a tart or acidic taste, "determined" means having a strong resolve or being motivated, and "physically" refers to something related to the body.
Therefore, the correct answer is D) strenuous.