The answer is C: 27.

The rule
Take away 3 each time.
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.
number = 45
for step in range(7):
print(number)
number = number - 3
When you run it, Python prints:
45 42 39 36 33 30 27
Pixel says: Try changing a number in the code and run it again. Can you make the pattern go on for 20 steps? Meow!