**Core Concept**
The root canal instrument most likely to break during use is a broach, which is a type of hand instrument used for shaping and enlarging the root canal. Broaches are typically made of a hard, rigid material that is prone to fracture under excessive force or stress.

**Why the Correct Answer is Right**
Broaches are more susceptible to breakage due to their design and material properties. They consist of a series of tines or flutes that are used to remove dentin and shape the canal. However, when subjected to excessive torque or force, the tines can bend or fracture, leading to instrument breakage. This is particularly common when using broaches to negotiate curved or narrow canals.

**Why Each Wrong Option is Incorrect**
**Option A:** Files, on the other hand, are designed for cutting and shaping dentin, and are generally less prone to breakage than broaches. While files can be brittle and prone to fracture, they are typically less likely to break during use than broaches.
**Option C:** Reamers are rotary instruments used to enlarge and shape the root canal. While they can be prone to breakage, it is less common than with broaches, and is often due to overheating or excessive wear.
**Option D:** Spreaders are used to separate the root canal walls and are generally less likely to break during use than broaches. They are typically designed for gentle manipulation and are less prone to fracture than broaches.

**Clinical Pearl / High-Yield Fact**
When using broaches, it is essential to maintain a gentle touch and avoid excessive force or torque to minimize the risk of breakage. Additionally, using broaches with a well-designed and maintained canal preparation can help reduce the likelihood of instrument breakage.

**✓ Correct Answer: B. Broach**