There are a couple of ways you can obtain approval information by a particular KB number.
One way is from the default All Updates view. Enable a filter on the KB IDs column for the particular KB article(s) that you are interested in reviewing, and then observe the values in the Approval and Approved for Computer Groups columns.
Another way is from the Update Approvals tab on the WSUS server node. Again, select the desired KB article(s) from the KB IDs column. Observe the values in the Action and Computer Group columns. The difference here is that the Update Approvals tab will only show updates that have had actual approval actions taken against them (Never Approved updates will not be displayed), but this tab also shows you when an update was approved (or Unapproved), and who did it.