I am trying to create a variable that assess the construct of "knowledge" using participant's responses to about 20 questions. These questions have different response options. Some of the questions have Yes/No options, while others have multiple response options where multiple answers are possible. Among the multiple response options, two of the options are "Other(specify)" and "Don't know". Other options in the multiple response questions range in number from 4 to 7 for different questions. I was wondering if the responses to these different questions can be combined to form one variable using something like factor analysis.

An example multiple response question would be:
What are some potential advantages of composting/?
Option 1 - Advantage one
Option 2 - Advantage two
Option 3 - Advantage three
Option 4 - Advantage four
Option 5 - Other(specify)
Option 6 - Don't know

I also want to know if anything changes if I am only using 4 questions/items as opposed to 20 to form the variable.