Final answer:
The slowest absorption in injection solutions is achieved with oil-based solutions, which have low water solubility.
Step-by-step explanation:
When it comes to injection solutions, the base that will result in the slowest absorption is oil-based solutions. Oil-based solutions are known for their low water solubility, which slows down their absorption into the bloodstream. For example, medications like testosterone or certain vitamins are often administered in oil-based solutions. The oil-based solution slows down the release of the medication into the bloodstream, resulting in a more prolonged effect. Other injection solutions, such as water-based or saline solutions, are more readily absorbed and therefore have a faster onset of action.