In terms of metric prefixes, how many millilitres are there in one litre?

One litre is equal to 1000 millilitres, which is the standard conversion in the metric system. The prefix "milli-" in the metric system indicates a factor of (10^{-3}) or one-thousandth. Therefore, when you multiply one litre (which is 1000 millilitres) by the conversion factor for millilitres, you arrive at the conclusion that there are indeed 1000 millilitres in one litre.

The other choices do not appropriately reflect this metric relationship. For instance, 100 millilitres and 500 millilitres are both significantly lower than a litre, and 1500 millilitres exceeds one litre, indicating a misunderstanding of how metric prefixes scale. Thus, the relationship of litre to millilitre supports the conclusion that 1000 millilitres equate to one litre.