**Core Concept**
Formalin, a solution of formaldehyde in water, is a widely used fixative in pathology and histology. It cross-links proteins and preserves tissue structure, making it an essential tool for diagnostic purposes. The exact concentration of formalin used can impact the fixation outcome.

**Why the Correct Answer is Right**
The correct answer is related to the concentration of formalin, which affects the fixation process. Formalin is typically used at a concentration of 10% formaldehyde in water, although the exact concentration may vary depending on the specific application. The fixation process involves the conversion of formaldehyde into methylene bridges, which cross-link proteins and preserve tissue structure. This process is crucial for maintaining tissue morphology and facilitating accurate histopathological diagnosis. The concentration of formalin is a critical factor in achieving optimal fixation, as under-fixation can lead to tissue disruption, while over-fixation can result in tissue hardening.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it is not a recognized concentration of formalin commonly used in pathology and histology. The use of a 5% formalin solution would likely result in under-fixation and tissue disruption.

**Option B:**
This option is incorrect because it is not a standard concentration of formalin used in formalin fixation. A 20% formalin solution would likely result in over-fixation and tissue hardening.

**Option C:**
This option is incorrect because it is not a recognized concentration of formalin commonly used in pathology and histology. The use of a 15% formalin solution would likely result in suboptimal fixation and tissue disruption.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the concentration of formalin used can impact the fixation outcome, and using the correct concentration is crucial for maintaining tissue morphology and facilitating accurate histopathological diagnosis.

**Correct Answer:** D.