Which prefix signifies the number 'one'?

The prefix that signifies the number 'one' is "uni-". This prefix is derived from Latin and is commonly used in medical terminology to indicate singularity or a onefold condition. For instance, in medical terms like "unilateral," it describes something that affects only one side of an organism or structure.

The other prefixes in the choices have different meanings; "dys-" typically indicates difficulty, impairment, or abnormality. "Chol/e-" relates to bile or the gallbladder, and "-plegia" refers to paralysis. These prefixes do not convey the concept of 'one,' making "uni-" the appropriate choice for signifying the number 'one.'