**Category:** Mechanics  
**Subtype:** Oscillations (Simple Harmonic Motion)  
**Problem Description:** The problem is about understanding the conditions under which the kinetic energy of a mass-spring system (a simple harmonic oscillator) reaches its minimum value.  

**Key Concepts:**  
- The kinetic energy is at its minimum when the speed of the mass is at its lowest.  
- This occurs when the spring is either fully compressed or fully extended, meaning the spring is at its maximum length (not deformed) or at its equilibrium position.  

**Question:** When does the kinetic energy of the mass-spring system reach its minimum?  
**Options:**  
A. The mass has maximum velocity.  
B. The spring is not deformed.  
C. The mass passes through the equilibrium position.  
D. The spring is at maximum length.  

**Correct Answer:** D. The spring is at maximum length.