The answer is D: green star.

The rule
The pattern repeats every 4 things: green diamond, green star, green hexagon, green star. 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 diamond", "green star", "green hexagon", "green star"]
for position in range(10):
print(position + 1, pattern[position % len(pattern)])
When you run it, Python prints:
1 green diamond 2 green star 3 green hexagon 4 green star 5 green diamond 6 green star 7 green hexagon 8 green star 9 green diamond 10 green star
Pixel says: Try changing a number in the code and run it again. Can you make the pattern go on for 20 steps? Meow!