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."
Explanation: This sentence expresses a casual and personal tone, which is characteristic of friendly correspondence. It indicates concern for the recipient's well-being and includes a familiar reference to "the crew," suggesting a close relationship. The other sentences are more formal in tone, with a focus on reporting or asking questions, which is less characteristic of a friendly letter.