Reduction of multiple answers to single variable
The variable represents the answer to the first question. 

One straightforward way is to allow for all possible categories in this variable. For example, if there are 5 options in this answer, you will have to treat it as a categorical variable with 2^5 = 32 categories. 

However, the number of categories increase exponentially with the number of options (check boxes) provided for the answer. In that case, it might be better to restrict the number of categories to, for example, 5. This can be done by leaving the top 4 choices/ options (by count) as they are and treating every other choice as "other".