Just a piece of advice: Always its a good practice if you can post the code for creating objects and data. That way, it will be easy for every one as we all do not have to create the objects and the data you are referring to.

Try this:

Select A.eventid, B.ackcomments,C.fixtime,D.completecomments, D.completetime

From Activity A

JOIN ActivtyAck B

ON A.activityID = B.activityID

JOIN Acitivtyfixed C

ON A.activityID= C.activityID

JOIN Activitycomplete D

ON A.activityID= D.activityID