This depends on the rest of your use case. If it's just a single question it might be enough to have a boolean attribute in one of the entities you've already created for each client. If you're looking to have multiple questions / answers, you could consider making it something like this:

• Make two entities:
  • Question with an attribute for the string that is your question
  • Answer with a boolean attribute for the answer

This makes the whole thing much more configurable so you can have multiple questions that are configurable front-end.

Enumeration attribute can be used to store answer with two options i.e., yes and no. Then for answer input you may use a drop down or radio buttons and you can also simply use a boolean attribute. For questions, you would se a string attribute and display its value to show the question. 
If you provide more details on your exact requirements, then a proper flow for your scenario can be suggested

Hello, Alvaro Riaño

Take attribute, set datatype as boolean or enumeration and in the widget, you can take a drop-down list or radio button to select "yes" and "No".

Hope this will help you.