The answer is D: green square.

The rule
The pattern repeats every 3 things: yellow square, green square, green square. 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 = ["yellow square", "green square", "green square"]
for position in range(8):
print(position + 1, pattern[position % len(pattern)])
When you run it, Python prints:
1 yellow square 2 green square 3 green square 4 yellow square 5 green square 6 green square 7 yellow square 8 green square
Pixel says: Try changing a number in the code and run it again. Can you make the pattern go on for 20 steps? Meow!