Step 1: Concept
This is a fundamental theorem stating that monotonicity is a sufficient condition for integrability.
Step 2: Meaning
A monotonic function (either always increasing or always decreasing) on a closed interval $[a,b]$ is always bounded by its values at the endpoints.
Step 3: Analysis
For any monotonic function, the difference between the Upper and Lower sums can be made arbitrarily small by refining the partition, satisfying the Riemann criterion.
Step 4: Conclusion
Therefore, every monotonic function on $[a,b]$ is integrable.
Final Answer: (C)