You can view respondents who unsubscribed and the invoked reason on the Contacts page. Go to the Contacts page, scroll through your customers listing page, and check if you have the specific yellow tag [UNSUBSCRIBED].
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In case you have a large audience and scrolling through the whole list takes a lot of time, you have the option to use the filter option and select only the Unsubscribed customers. In the right corner of your Contacts menu, just click on the Opted Out filter.

By hovering your mouse indicator on it you can also notice the invoked reason for unsubscribing.