Mongodb tree structure questions and answers
If the whole survey is not too big and if it will not grow, I would consider putting all the questions and answers inside one document.

So one document of that collection would look like

{
  "survey": {
    "Question A": [
      {
        "Ans A": [
          {
            "Question C": [...]
          }
        ]
      },
      {
        "Ans B": [
          {
            "Question D": [...],
            "Question E": [...]
          }
        ]
      }
    ]
  }
}


If it is big, and could grow in future, I would consider putting answer and question ids as key-value pair like following.

{
  "title": "Question A",
  "answers": [
    {
      "Ans A": [
        "Question C's Id"
      ]
    },
    {
      "Ans B": [
        "Question D's Id",
        "Question E's Id"
      ]
    }
  ]
}


I wouldn't normalize to have both question and answer collection, because it would take too much time to join each other.