Making independent groups of option buttons
You need to enclose each group of option buttons in a Group Box control. You can access this from the same menu you used to add the buttons.
See this link for more info: http://office.microsoft.com/en-us/excel-help/add-a-group-box-or-frame-control-to-a-worksheet-HP010236679.aspx.
I agree with Nixz's response and would like to add more detail. Nixz posted: A simple way to do is to set the GroupName property for the option buttons in the form. Set it to OB1 for first three option buttons and OB2 for the next three option buttons. Now they will work as two groups and you can check one each from eithr groups.
To make this work for me, I edited the GroupName in Properties for each radio button selection. For one group, I edited the two selections' GroupName(s) and called them each Question2. For the second set, I edited the GroupName for each radio button choice, calling them both Question5. This told excel they were grouped, even though they were in a Group Box, eXcel was not recognizing them as in separate Group Boxes. Hope this helps!