Arithmetic sequences have a common difference, while Geometric sequences have a common ratio.
In the sequence -20, -18, -15, -11..., there is not a common difference. This is because there is a +2 increase between -20 and -18 but a +3 difference between -18 and -15, then +4 increase between -15 and -11.
This means it is not Arithmetic, which means it can't be both either.
This leaves us with B and C, so we have to see if there is a common ratio.
To find common ratio, divide any 2 terms in the sequence.
Let's choose -18 and -15.
-18 / -15 = 1.2
Now let's see if this works for each term.
To do this, multiply each term by 1.2 to see if it results in the next term.
-20 • 1.2 = -24 This doesn't work, so there is no common ratio.
This means the sequence is neither Arithmetic nor Geometric.
So the answer is C. Neither.