Which term of the series 5, 10, 20, 40, ….. is 1280?

The correct answer is D. None of these.

The series 5, 10, 20, 40, ….. is a geometric series with a common ratio of 2. The general formula for a geometric series is $a_n = a_1 r^{n-1}$, where $a_n$ is the $n$th term, $a_1$ is the first term, $r$ is the common ratio, and $n$ is the number of terms.

In this case, $a_1 = 5$ and $r = 2$. We can use the formula to find the $10$th term:

$a_{10} = 5 \cdot 2^{10-1} = 2^{11} = 2048$

The $9$th term is:

$a_9 = 5 \cdot 2^{9-1} = 2^{10} = 1024$

The $8$th term is:

$a_8 = 5 \cdot 2^{8-1} = 2^{9} = 512$

Therefore, the $10$th term is not 1280, the $9$th term is not 1280, and the $8$th term is not 1280. Therefore, the answer is D. None of these.