The length of the diagonal brace when Jonathan is building a box in the shape of a rectangular prism is 17 inches.
From the information given, the dimensions are given as 8 inches by 12 inches by 9 inches.
Therefore, the length of the diagonal brace will be:
= ✓12 + ✓8 + ✓9
= ✓289
= 17
In conclusion, the correct option is 17 inches.
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