How to get form variables data into twig templates?
Put the custom variable in a key with a '#', so that the formbuilder doesn't try to render this as a form element.

buildForm():

  $form['#myvars']['var1'] = 'xyz';


In twig you can access the variable in element:

form.html.twig:

  {{ element['#myvars'].var1 }}


If you set a template in #theme of the form build 

buildForm():

  $form['#theme'] = 'take_mcq_form';


you get a second template inside of form.html.twig, where you can access the variable in form:

take_mcq_form.html.twig:

  {{ form['#myvars'].var1 }}