The answer is D: yellow hexagon.

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