What Was the Purpose of the "Signature Frames" Found at the Bottom

What was the purpose of the "signature frames" found at the bottom of many stained-glass windows?

A) to specify the Priest presiding over the church in which the window was housed
B) to specify the Bishop presiding over the cathedral in which the window was housed.
C) to indicate the date and craftsman who created the window
D) to designate the group or person who donated the window