Answer:
It is an Arithmetic series which is finite.
Explanation:
Given series,
-5, -10, -15, -20
∵ A series is called Geometric series if there is a constant ratio ( called common ratio ) in the consecutive terms,
While, it is called Arithmetic series if there is a constant difference in consecutive terms,

That is, it is not a GP,
Now, -10 + (-5) = -15 - (-10) = -20 - (-15)
Thus, it is an AP,
Since, a series having finite number of elements is called finite series otherwise it is called infinite series,
Here, the number of elements is 4 ( finite )
Therefore, the given series is finite.