According to the working memory theory of listening, people find it harder to listen when they are expected to remember everything they hear they are distracted by surrounding noise they are part of a large group their capacity for listening has been reached.

their capacity for listening has been reached.                                      

Explanation:

          According to the working memory theory of listening, when the listening capacity of the human being is finally reached, people find it hard to remember what they hears.

        It is theory where it becomes difficult to concentrate and remember what they hears when people's short term memory is reached or is full. It occurs when people have already heard many things and there is no practical no free space for new information to be remembered.

Hence the answer is ---

"their capacity for listening has been reached".  

could you show the options for the answers? : )