The answer is B: green circle.

The rule
The pattern repeats every 3 things: green square, green circle, green circle. Then it starts again from the beginning.
The pattern as Python code
Here is a tiny Python program that follows the same rule. Type it into IDLE and run it to print the whole pattern, including the answer.
pattern = ["green square", "green circle", "green circle"]
for position in range(8):
print(position + 1, pattern[position % len(pattern)])
When you run it, Python prints:
1 green square 2 green circle 3 green circle 4 green square 5 green circle 6 green circle 7 green square 8 green circle
Pixel says: Try changing a number in the code and run it again. Can you make the pattern go on for 20 steps? Meow!