Let's compelete the following set: A = {6, 12, 18,..., 42}
It appears that the set has a common difference of 6. Thus, we get:
A = {6, 12, 18,..., 42}
A = {6, 12, 18, 24, 30, 36, 42}
1 2 3 4 5 6 7
Therefore, the cardinal number of the given set is 7.
The answer is letter D.
