**Category:** Wave Physics  
**Subtype:** Wave Motion / Harmonic Waves  
**Problem Description:** This problem involves determining the equation of motion for a point \( M \) on a wave that is propagating from a source \( O \). The source has a specific oscillation equation, and we need to find the correct form of the wave equation for point \( M \), which is located a certain distance from the source.

**Key Information:**
- The source wave equation is given as \( u_O = a \cos(20\pi t) \) (where \( a \) is the amplitude).
- The speed of the wave is \( 100 \, \text{cm/s} \).
- The distance from the source \( O \) to point \( M \) is \( 2.5 \, \text{cm} \).

**Objective:** Identify the correct wave equation for point \( M \) based on the given parameters.