What would you like to ask someone about getting milk?

The use of the phrase "would you mind getting some milk on the way" is polite and conveys a clear request for the person to pick up milk. It implies that the task is not overly demanding and shows consideration for the other person's schedule, suggesting that it's an added favor on top of their current plans. This phrasing emphasizes both the request and the context of it being a favor.

In contrast, the other options lack the same level of politeness or clarity. Questions like "if they have time for milk" may sound more ambiguous, suggesting a focus on their time rather than explicitly asking for them to bring milk. "Could you please buy milk later" is a bit less graceful in its phrasing, while "do you want milk when you come" implies the person is coming over rather than requesting them to pick it up, which changes the nature of the request entirely. Thus, the second option stands out as the most effective way to ask someone to get milk.