Which suffix means "small" or "diminutive"?

The suffix that means "small" or "diminutive" is indeed -ule. This suffix is commonly used in medical terminology to denote a smaller version of something. For example, "cell" can become "cellule," indicating a smaller cell, or "glob" can become "globule," referring to a small globe or droplet.

The other suffixes listed do not convey the meaning of smallness. For instance, -rrhea refers to flow or discharge, -phobia denotes an irrational fear of something, and -ation indicates a process or condition. None of these relate to size or diminutive forms, which reinforces why -ule is the correct choice for expressing a smaller version of an object or concept in medical terminology.