Final answer:
The sentence that provides evidence to support the claim of incorporating journaling in medical treatment is option C, which directly links expressive writing to positive health outcomes.
Step-by-step explanation:
To support the claim that 'physicians should incorporate patient journaling as part of their treatment for any complaint or disease,' the author must provide strong and relevant evidence. Among the given options, the sentence that serves as evidence to support the claim is:
C "In addition to reducing symptoms of existing diseases, expressive writing can also strengthen the body's ability to fight off disease."
This sentence directly supports the claim by stating a benefit of patient journaling, linking it to improved health outcomes. In contrast, the other sentences do not provide direct evidence that journaling could be beneficial as part of a medical treatment plan. For instance, sentence A actually presents a counterpoint, sentence B describes a mechanism but not a health outcome, and sentence D points to a specific study but does not generalize the benefit to all diseases and complaints.