Final answer:
"PO" is an abbreviation in the medical field that stands for 'by mouth', indicating oral medication administration.
Step-by-step explanation:
"PO" stands for 'by mouth', and it is an abbreviation commonly used in the medical field, particularly in prescriptions. When a medication is prescribed to be taken "PO", it means that it should be taken orally. Other options mentioned, such as milliliter (mL), with, and hour, are not represented by "PO". The milliliter is a unit of volume, "with" is a preposition, and "hour" is a unit of time.