The answer is D: green diamond.

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