**Core Concept**
Ni-Ti (Nickel-Titanium) instruments are a type of alloy used in endodontics, known for their unique properties that make them suitable for various dental applications. The specific property being tested in this question is the flexibility of Ni-Ti instruments.

**Why the Correct Answer is Right**
The flexibility of Ni-Ti instruments is primarily due to their high nickel content, which allows them to undergo significant elastic deformation without breaking. This is attributed to the shape-memory effect (SME) and superelasticity (SE) properties of Ni-Ti alloys. The SME enables Ni-Ti to return to its original shape after deformation, while SE allows it to absorb and distribute stresses without failing.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect. While Ni-Ti instruments are resistant to corrosion, this property does not contribute to their flexibility.
**Option B:** Incorrect. The high strength of Ni-Ti instruments is a separate property that does not directly relate to their flexibility.
**Option C:** Incorrect. The biocompatibility of Ni-Ti instruments is an essential property, but it does not explain their flexibility.

**Clinical Pearl / High-Yield Fact**
When working with Ni-Ti instruments, it's essential to remember that their flexible nature can make them more prone to fatigue, especially when subjected to repeated bending or torsion. Regular maintenance and storage can help prevent damage to these instruments.

**Correct Answer:** D. Their unique combination of the shape-memory effect and superelasticity properties allows Ni-Ti instruments to exhibit flexibility.