Answer:
I (capital i).
Step-by-step explanation:
We have the pattern:
CIRCUSCIRCUSCIRCUS...
The word "CIRCUS" repeats indefinitely.
And we want to find the 500th letter.
Note that CIRCUS repeats after every 6th letter. So, it will repeat and go back to C after 6 letters, 12 letters, 18 letters, and so on...
Note that 500 is the same as:

So, 500 letters will be the word CIRCUS repeated 83 times plus 2 extra letters.
So the first 83 iterations won't matter as it'll just repeat. We only need to focus on the remaining letters.
After 500 letters, we will have reiterated CIRCUS 83 times for a total of 498 letters and we will start back at C at the 499th letter.
So, our 500th letter will be the letter after (the first) C, or I.
Our 500th letter is I (capital i).
And we're done!