In finding the answer choice that strengthens the argument in this question, it should be noted that the answer must be directly relevant to the specific argument in question.

Furthermore, it must also provide information that justifies the argument (in this case, To drink a truly healthy herbal tea, the ingredients should be collected from a natural source.)

The premise that led to the argument conclusion: An investigation showed that out of the 7 brands of tea sold at an average-sized supermarket, all included preservatives and added chemicals for flavor and color.

Option A - Incorrect
This answer choice tells us about the increase in the popularity of herbal tea which may not have an impact on determining a truly healthy herbal tea for consumption.

Option B - Incorrect
This is too general as it discusses the consideration for product sales in average-size supermarkets, which is irrelevant to the passage argument.

Option C - Correct
This claim provides a relevant point that justifies the passage argument in the sense that, herbs grown with water alone (i.e natural source) gives a more "healthy" herbal tea as it doesn't contain food-industry chemicals that have been proven to disrupt digestion.

Option D - Incorrect
This doesn't provide enough relevant information to strengthen this argument

Option E - Incorrect
This answer choice provides a claim that weakens the argument rather than strengthens it. In addition, the fact that we can't rule the possibility of the tea herbs from farms not having residue from chemicals (pesticides) or hormones discredit this option the more.