Yes/No is a ready-made question with the answers Yes and No. It needs no answer options. Use it for clear either-or questions, and as a filter: show follow-up questions only to participants who answered Yes.
When the question is not mandatory, participants can also pick No answer.
Click the purple + in the top bar. The question type picker opens.
Under Ranking & rating, click Yes/No. The question appears at the end of the current question group.
Click Your question here and type your question.
Select the question to open its settings on the right.
The answer is Y for Yes and N for No, and empty for No answer. In a condition, a question with the code Q015 answered with Yes is Q015 == "Y".
Not to this type: its answers are fixed. Use List (radio) or Single choice buttons with the answers Yes, No and Don't know.
Select the follow-up question and add a condition in the Condition designer of its settings, for example Q015 == "Y". See Conditions: show a question only when it applies.