I'm totally confused on the type of bulb needed for the Pentair Spa Light 77310100 AQ 100W 120V. The choices online are 5, 5A, 5B, 5C. I have never changed out the bulb so I am unsure  of the correct fitment. I guess I could open it up and see before ordering but I was just hoping I would only having to open it once for this job.  Any feedback would be appreciated.

https://www.inyopools.com/spa_parts_american_products_spabrite_aqualight.aspx