If you are allowed to bring a carry-on bag, what size is suitable?

If you are allowed to bring a carry-on bag, what size is suitable?
A.50cm*34cm*22cm (20”)
B.68cm *42cm *26cm (24”)
C.78cm *47cm *28cm (28”)
D.88cm*53cm*30cm (32”)
正确答案：50cm*34cm*22cm (20”)