We know that the chord is 25 units long.
Also, according to the image, side 18 and half of the given chord from a right triangle. Let's find the hypothenuse with Pythagorean's Theorem.
![\begin{gathered} r^2=18^2+25^2 \\ r=\sqrt[]{324+625}=\sqrt[]{949} \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/e6gsfwcv74izuaevbdembr29ztrz842g29.png)
Now, we use Pythagorean's theorem to find half of the chord x.
![\begin{gathered} (\sqrt[]{949})^2=((x)/(2))^2+18^2 \\ 949-324=((x)/(2))^2 \\ 625=((x)/(2))^2 \\ (x)/(2)=\sqrt[]{625}=25 \\ x=2\cdot25=50 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/s4ltimmpy4ln4hphl8oba5p3yk7gog6qy5.png)
Therefore, x is 50 units long. A is the right answer.