**Core Concept**
The **mitral valve** is a complex structure that ensures blood flows from the **left atrium** to the **left ventricle** during diastole and prevents backflow during systole. The proper functioning of the mitral valve is crucial for maintaining efficient cardiac output.

**Why the Correct Answer is Right**
The correct answer is related to the structure that plays a crucial role in preventing the mitral valve cusps from regurgitating into the left atrium during systole. This structure is the **chordae tendineae**, which are fibrous strings that connect the mitral valve cusps to the **papillary muscles**. During systole, the papillary muscles contract, tightening the chordae tendineae and preventing the mitral valve cusps from prolapsing into the left atrium.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not directly prevent mitral valve regurgitation.
**Option B:** This option is also incorrect as it is not directly involved in the prevention of mitral valve regurgitation.
**Option C:** This choice is incorrect because, although it is part of the mitral valve apparatus, it is not the primary structure that prevents regurgitation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **mitral valve apparatus**, including the chordae tendineae and papillary muscles, works together to ensure proper valve function. Damage to any part of this apparatus can lead to mitral regurgitation.

**Correct Answer:** D. Chordae tendineae.