Which suffix indicates a relationship to or characteristic of?

The suffix that indicates a relationship to or characteristic of is -tic. This suffix is commonly used in medical terminology to describe conditions, processes, or characteristics related to a specific aspect of health or disease. For example, the term "cardiotonic" refers to a drug or substance that affects the heart's function, highlighting its relationship to cardiac activity.

The other suffixes represent different concepts: -tomy refers to incision or cutting into a part of the body, -plasty signifies surgical repair or reconstruction, and -pathy denotes a disease or disorder. Therefore, -tic specifically conveys the idea of a characteristic or relation, making it the correct choice in this context.