The answer is D: yellow circle.

The rule
The pattern repeats every 4 things: pink triangle, yellow circle, pink triangle, orange 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 = ["pink triangle", "yellow circle", "pink triangle", "orange diamond"]
for position in range(10):
print(position + 1, pattern[position % len(pattern)])
When you run it, Python prints:
1 pink triangle 2 yellow circle 3 pink triangle 4 orange diamond 5 pink triangle 6 yellow circle 7 pink triangle 8 orange diamond 9 pink triangle 10 yellow 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!