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Answer:
C. 3√2 units
Explanation:
The diagonal of a square is √2 times the side length. For a square with side length 3, the diagonal is 3√2.
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If you like, you can see this using the Pythagorean theorem.
d² = s² + s² . . . . . where d is the diagonal, and s is the side
d² = 2s²
d = √(2s²) = s√2
For s=3, ...
d = 3√2