What does the tattoo need to consist of to indicate sterilization?

The tattoo that indicates sterilization must consist of a universal symbol for male or female with a slash through it. This symbol provides an immediate visual cue regarding the animal's reproductive status, which is critical information for both veterinary professionals and pet owners. The use of a universally recognized symbol helps ensure that anyone who encounters the animal can quickly identify that it has been spayed or neutered, thereby preventing unnecessary breeding and contributing to responsible pet ownership.

In regard to the other options: while initials of the veterinarian and a unique identification number might be useful for record-keeping purposes, they do not provide clear and immediate information about the animal's sterilization status. Similarly, colors representing different procedures lack a standardized meaning and would not offer the necessary clarity required for quick identification. Thus, the universal symbol with a slash is the most effective method for indicating that an animal has been sterilized.