Which of the following knots is commonly used for its secure hold but is not rated for efficiency?

The back splice is often used in various rope applications, particularly in situations involving termination of rope ends or creating a loop that won't unravel. However, it's important to recognize that while it provides a secure hold and enhances the durability of the rope by adding bulk to the end, it is not classified with a specific efficiency rating.

Efficiency ratings for knots typically refer to how much strength is retained in the rope after the knot is tied; many knots reduce the overall strength of the rope significantly after being tied. The back splice, being more of a termination method rather than a knot meant for load-bearing applications in the same manner as others listed, doesn't fall into the category where efficiency is typically measured or rated.

Understanding that the back splice secures the end of the rope and prevents fraying without being intended for load-bearing scenarios clarifies why it lacks an efficiency rating while still serving a significant and practical purpose in rope management and use.