The length of arc AB the minor arc is 40 cm what is the circumference of circle C?

All you've told us is that 40 cm is less than 1/2 of the circumference.

With that information, all we know is that the circumference is more than 80 cm.

We could calculate it if we knew what the angle is at the center of circle

between the two radii (radiuses) that go to the ends of arc AB. We're

guessing that it's there in your book, but you forgot to include it when

you decided to ask us to do your homework problem for you.