Answer:
1). 'Customer' sharing has not been checked on the Knowledge Article.
Explanation:
As per the question, the key reason for the articles not being visible to the external users is that 'the 'Customer' sharing has not been checked on the Knowledge Article' and thus, the external customers are unable to read that article. The article needs to enable customer sharing of the document i.e. article to ensure its availability and visibility to all the customers/customer community so that they can have access to the knowledgeable resource. Therefore, option 1 is the correct answer.
