Answer: Option 'a' is correct.
Step-by-step explanation:
Since we have given that
99, 89.9, 78.8, 68.7, . . . . .
Here,
But we can see that there is minor difference between the consecutive terms:
So, we will go for common difference whether it is same or not.
Their difference are not common.
So, It is neither geometric nor arithmetic.
Hence, Option 'a' is correct.