A dilation that decreases the size of the original object must have a scale factor of less than 1.
Let's analyze the scales provided:
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Figure 1: Star with scale 1:5 - This indicates that the star is being represented at 1/5th the size of the original, which is a reduction. Scale factor = 1/5 = 0.2.
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Figure 2: Octagon with scale 2:0.5 - This implies the octagon is represented at double its size (2) divided by half (0.5) which equals 4 (not a reduction).
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Figure 3: Triangle with scale 10:1 - This means the triangle is being represented at 10 times its original size (not a reduction).
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Figure 4: Pentagon with scale 1:0.1 - This represents the pentagon at a scale that is 1/10th of its size, which is also a reduction. Scale factor = 1/0.1 = 10 (again, not a decrease).
From this analysis, Figure 1 (the star with scale 1:5) is a dilation that decreases the size of the original object it represents.