How many mL are required to completely fill a prescription if 342 mL is the provided amount?

To determine how many milliliters are required to completely fill the prescription, one must assess the provided amount, which is 342 mL. If the prescription is for 342 mL, then the entire volume needed to fulfill that prescription is indeed 342 mL. Since the question asks for the amount required to completely fill it, and no other quantities or conditions are introduced, the answer logically remains 342 mL, as this is the exact measurement provided and needed.

In this scenario, the focus is on matching the amount required with what is provided. Other options, such as 500 mL, 400 mL, and 300 mL, do not correspond with the required measurement of 342 mL, as they exceed or fall short of the prescribed quantity. Therefore, the answer is based straightforwardly on the stated need for 342 mL to fully satisfy the prescription requirement.