What are some good practices for a supervisor?
As in any teaching situation, your job is to educate, not just watch over the student. This means, that you have some responsibility that the student continue to make progress toward their goals. You need to assure that they don't get stuck, or, rather, that they don't stay stuck for long. 

This will take some time and interaction - regular meetings, reading their work, giving feedback, etc. 

But, you don't have to give a direct answer to any particular question. What you need to do is to assure that the student has a way to find the answer, even if it doesn't come directly from you. Answer questions with other questions, for example, so that the student can have her/his own insights. 

In the example, you give, it is better, IMO, to ask the student to tell you the tradeoffs of each method, A and B. You don't need to tell them to "go away and research it". Tell them to lay out the pros and cons of each method. If they miss something in their explanation, you can help them along, of course. 

But if they have no idea of the tradeoffs, then you should probably send them off on a quest of learning. 

The short form is "give them the minimum they need to keep moving generally forward". This lets them develop confidence over time, make the work their own, and, still, not get (or stay) stuck. 

If done well, this will also put the student on the path to intellectual independence, which they will need as they continue.