Understanding Speed Units
Speed is a measure of how quickly an object moves, expressed as distance covered over a specific time. The basic unit of speed in the International System of Units (SI) is meters per second (m/s).

Why m/s is the Basic Unit:
- **SI Units:**
- The SI unit system is widely accepted for scientific and mathematical calculations.
- In this system, the base unit of distance is the meter (m) and the base unit of time is the second (s).
- **Definition of Speed:**
- Speed is defined mathematically as:
- Speed = Distance / Time
- Using SI units, this translates to:
- Speed (in m/s) = Distance (in meters) / Time (in seconds)

Comparison with Other Options:
- **km/min (Option A):**
- While valid, it is not a standard SI unit and can lead to confusion in scientific contexts.
- **m/min (Option B):**
- This unit is less commonly used and not as intuitive for scientific calculations as m/s.
- **km/h (Option C):**
- Commonly used in everyday contexts (like road signs), but still not the basic SI unit.

Conclusion:
The choice of meters per second (m/s) as the basic unit of speed is due to its alignment with the SI unit system, making calculations consistent and straightforward for scientific purposes. Thus, option 'D' (m/s) is indeed the correct answer.