Both 은/는 and 이/가 can be used in that sentence, but they'll have different connotations. 1. 이름은 무엇입니까? I know many things about the person, but now I want to know the name. 2. 이름이 무엇입니까? I want to know the person's name (without any special nuance). 은/는 is called the topic marker. It sets the subject as the topic and explain and elaborate more about it. It can also have the nuance of contrasting the topic against other things, as a particularly strong interest in something can mean less interest in other things of the same group. 이/가 doesn't have such connotations. It just mentions a fact of interest, usually because it is needed in some way. So, if you're commenting about a cute boy to his mother, you might ask 이름은 뭐예요?, as if you want to know the name this time in addition to other things you already know about him. On the other hand, if you are requesting something at a government office, the clerk there will ask 이름이 뭐예요? since they just need it for the work at hand and don't really care about you as a topic. Here's a link to a question of the same topic: https://www.italki.com/question/405436

Andreas, I watched the video. Thanks a lot.

Talk to me in Korean made a video about this. http://talktomeinkorean.com/lessons/topic-subject-markers/