**Core Concept**
The question tests the understanding of the relationship between the nib diameter and compaction force in a pharmaceutical or materials science context, specifically in tablet formation or powder compaction. This involves **pharmacological** and **physiological** principles related to **drug delivery systems**. The nib diameter refers to the diameter of a punch or die used in compressing powders.

**Why the Correct Answer is Right**
When the nib diameter is reduced by half, the compaction force increases because the same amount of force is applied over a smaller area, thus increasing the **pressure**. This is based on the principle that pressure (P) is equal to force (F) divided by area (A), P = F/A. By reducing the area (in this case, by reducing the diameter of the nib), the pressure or compaction force increases if the force applied remains constant.

**Why Each Wrong Option is Incorrect**
**Option A:** This option would be incorrect because reducing the nib diameter does not directly reduce the compaction force; instead, it increases the pressure due to the decreased area over which the force is applied.
**Option B:** This option is also incorrect as the relationship between nib diameter and compaction force is not directly proportional in a way that would suggest the compaction force remains the same when the diameter is halved.
**Option C:** This option is incorrect because the compaction force does not decrease when the nib diameter is reduced; the force applied per unit area (pressure) actually increases.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **pressure** exerted during compaction is inversely related to the area of the nib or punch. This principle is crucial in pharmaceutical tablet manufacturing, where controlling the compaction force is essential for producing tablets of the desired hardness and dissolution characteristics.

**Correct Answer:** Correct Answer: D.