A region $ S$ in the complex plane is defined by \[ S \equal{} \{x \plus{} iy: \minus{} 1\le x\le1, \minus{} 1\le y\le1\}.\] A complex number $ z \equal{} x \plus{} iy$ is chosen uniformly at random from $ S$. What is the probability that $ \left(\frac34 \plus{} \frac34i\right)z$ is also in $ S$?

$ \textbf{(A)}\ \frac12\qquad \textbf{(B)}\ \frac23\qquad \textbf{(C)}\ \frac34\qquad \textbf{(D)}\ \frac79\qquad \textbf{(E)}\ \frac78$
1. We start by analyzing the transformation applied to the complex number \( z = x + iy \) when multiplied by \( \left(\frac{3}{4} + \frac{3}{4}i\right) \).

2. Express \( \left(\frac{3}{4} + \frac{3}{4}i\right) \) in polar form:
   \[
   \left(\frac{3}{4} + \frac{3}{4}i\right) = \frac{3\sqrt{2}}{4} \left(\cos\frac{\pi}{4} + i\sin\frac{\pi}{4}\right)
   \]
   This indicates a rotation by \( \frac{\pi}{4} \) radians and a scaling by \( \frac{3\sqrt{2}}{4} \).

3. To determine the effect on the region \( S \), we need to consider the transformation of the boundaries of \( S \). The region \( S \) is defined by:
   \[
   S = \{x + iy : -1 \le x \le 1, -1 \le y \le 1\}
   \]

4. When \( z \) is multiplied by \( \left(\frac{3}{4} + \frac{3}{4}i\right) \), the new region is scaled by \( \frac{3\sqrt{2}}{4} \) and rotated by \( \frac{\pi}{4} \). To find the permissible points, we need to consider the inverse transformation:
   \[
   \text{Inverse scaling factor} = \frac{4}{3\sqrt{2}} = \frac{2\sqrt{2}}{3}
   \]

5. The original square \( S \) is scaled by \( \frac{2\sqrt{2}}{3} \) and rotated by \( -\frac{\pi}{4} \). The new region must fit within the original square \( S \).

6. The area of the transformed region is:
   \[
   \left(\frac{2\sqrt{2}}{3}\right)^2 = \frac{8}{9}
   \]

7. The total area of the original region \( S \) is:
   \[
   2 \times 2 = 4
   \]

8. The probability that a randomly chosen \( z \) from \( S \) will result in \( \left(\frac{3}{4} + \frac{3}{4}i\right)z \) also being in \( S \) is the ratio of the area of the permissible region to the area of \( S \):
   \[
   \frac{\frac{28}{9}}{4} = \frac{28}{36} = \frac{7}{9}
   \]

Conclusion:
\[
\boxed{\frac{7}{9}}
\]