Step 1: Understand the reaction.
Manganese dioxide (MnO\(_2\)) dissolves in water to form Mn\(^{2+}\), which imparts a red color to the solution.
Step 2: Conclusion.
Therefore, the color of the acid formed when MnO\(_2\) dissolves in water is red.
Final Answer:
\[
\boxed{\text{D) red}}
\]