According to general rule, when the nonrestrictive or nonessential information is found within the sentence, rather than at the end of it, it should be set off using a pair of commas. In the given sentence, the words 'who usually follow strict nutritional guidelines' should be set off using commas on either side, as it can be termed as non-restrictive information.
Thus, the correct answer will be Option A.
A dash is used to denote an unexpected or emphatic pause.