The attached picture shows the Rapid City Regional Airport, KRAP. If I arrive after KRAP's tower is closed, which frequency should I use: 125.85 or 122.95?
You use the CTAF for announcing your position and intentions, i.e. 125.85. From the AIM 4-1-9(g):
If an airport has a tower and it is temporarily closed, or operated on a part-time basis and there is no FSS on the airport or the FSS is closed, use the CTAF to self-announce your position or intentions.
The CTAF may be a UNICOM, MULTICOM, FSS, or tower frequency and is identified in appropriate aeronautical publications.
125.85 is marked as the CTAF on the sectional and listed in the Chart Supplement.
Note: The letter "C" in the dark circle next to the control tower ("CT") frequency indicates that the CT frequency is the CTAF frequency at that airport.