Step 1: Concept
A particle is "at rest" when its velocity $v = \frac{ds}{dt} = 0$.
Step 2: Meaning
Differentiate $s = t^2 - 6t + 10$ with respect to $t$.
Step 3: Analysis
$v = 2t - 6$. Set $2t - 6 = 0 \implies 2t = 6 \implies t = 3$.
Step 4: Conclusion
The particle is at rest at $t = 3$ seconds.
Final Answer: (D)