Answer: 3. a human body; physique
Step-by-step explanation:
When a person's frame is being referred to, it is their physique or body that is the subject of interest. In saying that they kept craning their neck around their uncle's frame, the speaker is saying that they kept looking around from behind the body of their uncle.
Another example for instance is: John Cena has an imposing frame. This means that John Cena has a physique that can make people back down from trying to comfort him.