A. [‘da’, ‘ea’, ‘fa’, ‘db’, ‘eb’, ‘fb’, ‘dc’, ‘ec’, ‘fc’]
B. [[‘ad’, ‘bd’, ‘cd’], [‘ae’, ‘be’, ‘ce’], [‘af’, ‘bf’, ‘cf’]]
C. [[‘da’, ‘db’, ‘dc’], [‘ea’, ‘eb’, ‘ec’], [‘fa’, ‘fb’, ‘fc’]]
D. [‘ad’, ‘ae’, ‘af’, ‘bd’, ‘be’, ‘bf’, ‘cd’, ‘ce’, ‘cf’]

Solution

B. [[‘ad’, ‘bd’, ‘cd’], [‘ae’, ‘be’, ‘ce’], [‘af’, ‘bf’, ‘cf’]]

Explanation

The inner list is generated once for each value of j.