A minor arc is an arc that measures less than 180º
The minor arcs are: arc AB
A semicircle is an arc that measures exactly 180º
The semicircles are: arc ABC and arc ADC
A major arc is an arc that measures more than 180º
The major arcs are: arc ADB
To find all the major arcs, I looked at the minor arcs and stated the alternate way to connect those two points. Notice that the first and last letters are the same.