What is the meaning of 'Ringer' in relation to dogs?

The term 'Ringer' in relation to dogs refers to a substitute resembling another dog. This usage typically arises in contexts where a dog has been substituted for another, often with the intent of passing off the substitute as the original. For example, in some competitive or breeding situations, a "ringer" may be used to enhance the performance or pedigree claims of a participant or entry. The emphasis lies in the resemblance of the substitute to the original, which can create issues of authenticity or fairness in competitions.

The other options do not correctly define 'Ringer' as they pertain to different concepts within the realm of dog training and identification, such as agility training, specific types of collars, or classifications of dog breeds.