The sentence that would most likely be found in a letter that uses a friendly structure is:
"I just thought I’d check in to see how you and the crew are doing."
Reason: This sentence conveys a casual and warm tone, expressing interest in the recipient's well-being, which is typical of friendly communication. The use of phrases like "check in" implies a personal connection and friendliness, making it suitable for informal correspondence. In contrast, the other options are more formal or focused on specific topics, which is less characteristic of a friendly letter.