The answer is B: red diamond.

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