Final answer:
Barry should use the abbreviation "mm" to represent millimeters when labeling a diameter of 5 millimeters in his drawing.
Step-by-step explanation:
When Barry is drafting a top view of an object and wants to represent a diameter of 5 millimeter in the drawing, he should symbolize "millimeter" with the correct abbreviation. The standard abbreviation for millimeters is "mm". Therefore, the correct way to represent a 5 millimeter diameter on his drawing would be to label it as "5 mm".