Final answer:
The answer is 'body'. A hair form should be in proportion to the shape of the head, face, neck, and body line.
Step-by-step explanation:
The answer to the question is body.
A hair form should be in proportion to the shape of the head and face, the length and width of the neck, and the body line. Proportion refers to the relationship of parts of a body or form to one another and of the parts to the whole. In this case, the size and shape of the hair form should be in proportion to the body, ensuring that it harmonizes well with the overall appearance.