Answer:
D. 180 mm
Explanation:
At the bottom of the shape, we can see the base is 60mm. The next side(left or right) is shown as 20mm. The next side right before the missing piece is 20mm. The width of the missing piece is 10mm. The length of the missing piece is 20mm because:
20 + 20 = 40
60mm - 40mm = 20mm
60mm + 20mm + 20mm + 10mm + 20mm + 10mm + 20mm + 20mm
180 mm