Step 1: Concept
The function $|x-a|$ is continuous everywhere but not differentiable at $x=a$ because of the sharp corner (cusp) in its graph.
Step 2: Analysis
In $f(x) = 10 - |x-5|$, the absolute value part is $|x-5|$.
Step 3: Conclusion
The function has a "corner" at $x-5=0$, which is $x=5$. Therefore, it is not differentiable at $x=5$.
Final Answer: (D)