Final answer:
The content of the personal testimony that Summer emailed to Sarah is not provided in the question, so the answer must be 'd. Not provided'.
Step-by-step explanation:
The question relates to what personal testimony Summer provided in her email to Sarah. However, without additional context or information, such as the actual content of the email or the subject line, it's impossible to determine the nature of the personal testimony. Since the options given are relationship advice, cooking tips, travel recommendations, and 'not provided,' the correct choice based on the information provided in the question is d. Not provided. It's important to read carefully and gather as much context as possible when trying to understand correspondence like an email, but without the necessary details, one must conclude that the nature of the personal testimony is not known.