how to create a nested choice fields.

I want to create a question whith  nested options and not see the field option in the forms

Do you mean something like this example below with option groups inside a main option? 

<select>

  <optgroup label="Options 1">

     <option>Option 1.1</option>

     <option>Option 1.2</option>

  </optgroup>

  <optgroup label="Options 2">

     <option>Option 2.1</option>

     <option>Option 2.2</option>

  </optgroup>

</select>

If that's not what you meant then could you be a little more specific or provide us with an example of what it is you seek for this?