To rotate a point \((x, y)\) by \(270^\circ\) counterclockwise about the origin, you can use the transformation rule:
\[ (x, y) \to (y, -x) \]
For the point \((5, 8)\):
- Substitute \(x = 5\) and \(y = 8\) into the transformation: \[ (5, 8) \to (8, -5) \]
Thus, the new point after rotating \((5, 8)\) by \(270^\circ\) counterclockwise about the origin is \((8, -5)\).
Therefore, the correct answer is:
\((8, -5)\)