Simple to do my Friend.
Use "Modify" tab then select "Join" Command, click on end of arc then "Right Click" and "Close" command will appear, Click on "Close" and the Arc is converted to a Circle.
View Tutorial and Video Files for a step by step guide.
Give me a shout if you need assistance.
First create arc or open drawing containing arc
Go to "Modify" Tab select the "Join" command
Click on one end of Arc then "Right Click" and "Close" command will appear, click on "Close" command.
The Arc is then converted to a Circle