Removing Radius Dimension Arc
When using Radius dimension on an inside arc AutoCad will add an arc Extension line as shown below.
To remove the arc you need to make changes to the Dimension Style.
Go to Annotation, then Dimension Style.
Select New in the Dimension Style Manager.
Select Radius Dimensions then Continue.
Under the Lines tab check the Ext Line 1 box and select OK.
You will now have both a Standard style and a Radial style just for Radius dimensions. Select Close.
Now when you Radius dimension it will no longer have the arc, see below: