Which of the following decreases in motion along a straight line with constant retardation?

Options

A. Speed
B. Acceleration
C. Displacement
D. None of these

Explanation

To solve the question "Which of the following decreases in motion along a straight line with constant retardation?" we need to analyze the properties of motion under constant retardation. The options given are speed, acceleration, displacement, or none of these.

### Step-by-Step Solution:

1. **Understanding Retardation**:
- Retardation is defined as negative acceleration, which means it acts in the opposite direction to the motion of the object. This causes the object to slow down.

2. **Analyzing Speed**:
- When an object is under constant retardation, its speed decreases over time. For example, if an object starts with an initial speed \( u \) and experiences constant retardation, its speed at any later time \( t \) can be calculated using the equation:
\[ v = u - at \]
where \( a \) is the magnitude of retardation. As time progresses, \( v \) (the speed) will decrease until it reaches zero.

3. **Analyzing Acceleration**:
- In the case of constant retardation, the acceleration is constant and negative. It does not decrease; it remains the same throughout the motion. Therefore, acceleration does not decrease.

4. **Analyzing Displacement**:
- Displacement is the distance moved in a particular direction. Even though the speed of the object is decreasing, the displacement continues to increase until the object comes to rest. Thus, displacement does not decrease.

5. **Conclusion**:
- From the analysis above, we can conclude that the only property that decreases when an object is in motion along a straight line with constant retardation is **speed**.

### Final Answer:
The correct answer is **speed**.

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