The answer is D: pink square.

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