The answer is B: pink diamond.

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