**Core Concept**
The torsional limit refers to the maximum rotational force or torque that can be applied to an endodontic instrument, such as a file, before it fails due to excessive stress. This concept is crucial in endodontics, as it helps dentists avoid instrument breakage and ensure successful root canal treatments.

**Why the Correct Answer is Right**
The torsional limit is directly related to the instrument's ability to withstand rotational forces without failing. When an instrument is "locked" into place, it means that it has reached a point where it cannot be easily removed or rotated. At this point, applying further rotational torque will ultimately lead to instrument breakage. This occurs because the instrument's material properties, such as its yield strength and modulus of elasticity, are exceeded, causing it to deform plastically and eventually fail.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the torsional limit is not directly related to the apical pressure applied to the instrument. While apical pressure can affect the instrument's performance, it is not the primary factor determining the torsional limit.

**Option B:** This option is incorrect because the beginning of plastic deformation is actually related to the yield strength of the instrument, not the torsional limit. The torsional limit is the point at which the instrument fails due to excessive rotation, whereas the beginning of plastic deformation is an earlier stage in the instrument's deformation process.

**Option D:** This option is incorrect because the amount of force necessary for an instrument to not return to its original shape upon unloading is related to its elastic limit, not the torsional limit. The torsional limit is specifically concerned with the instrument's ability to withstand rotational forces, whereas the elastic limit is related to its ability to withstand compressive or tensile forces.

**Clinical Pearl / High-Yield Fact**
When working with endodontic instruments, it's essential to be aware of the torsional limit to avoid instrument breakage and ensure successful root canal treatments. A good rule of thumb is to use a gentle, controlled motion when rotating instruments, and to avoid applying excessive torque, especially when the instrument is "locked" into place.

**✓ Correct Answer: C. Amount of rotational torque that can be applied to a “locked” instrument to the point of breakage**