Final answer:
The common difference of the series is -6.
Step-by-step explanation:
The series in question is: 10 + 4 - 2 - 8 + ...
To find the common difference, we can look for a pattern in the series. Adding or subtracting the previous term consistently will give us the common difference.
By examining the series, we can see that each term is being subtracted by 6:
10 - 6 = 4, 4 - 6 = -2, -2 - 6 = -8, and so on.
Therefore, the common difference of the series is c. -6.