## Core Concept
The formation of the spindle apparatus is a crucial event in the mitosis of eukaryotic cells. The spindle apparatus is responsible for separating chromosomes into the daughter cells. It is composed of microtubules, which are dynamic structures made of tubulin proteins.

## Why the Correct Answer is Right
The correct answer, **Microtubules**, are indeed the components that form the spindle fibers. During cell division, particularly in the prophase of mitosis, the centrioles (located near the nucleus) begin to move apart. As they move apart, microtubules grow from the centrioles, eventually forming the spindle apparatus. This structure is essential for the proper segregation of chromosomes into the two new daughter cells.

## Why Each Wrong Option is Incorrect
- **Option A:** Centrioles are involved in the formation of the spindle apparatus as they are the sites from which microtubules grow, but they do not form the spindle itself.
- **Option B:** Chromatin is the complex of DNA and proteins in the nucleus, which condenses into chromosomes during mitosis. It does not directly form the spindle.
- **Option C:** Microfilaments are part of the cytoskeleton and are involved in cell contraction and movement but are not primarily responsible for forming the spindle apparatus.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the spindle apparatus's formation and function are critical for maintaining genetic integrity during cell division. Disruptions in this process can lead to aneuploidy, a condition where cells have an abnormal number of chromosomes, which is a hallmark of cancer cells.

## Correct Answer Line
**Correct Answer: D. Microtubules**