I am removing/eliminating a switch and light from upstairs and traced the wire to a basement junction box. The white wire from that romex is connected to a single red wire from circuit box. If I remove the romex from the old switch, what do I do with the red wire? Can I cap it or should I connect it to another set If wires in the junction box?
Remove separate the black wire from the others, the ground (bare copper) from the other ground wires, and white wire from the red wire. Re-use the red wire nuts on the resulting wires.
I have marked up your photo for clarity. The red wire is connected to the white wire (marked with yellow arrow). The hot (black) wire is marked by the orange arrow. The blue arrow points to the molex you are removing.
When you are reusing the red wire nut on the now dangling red wire, you may need to get a smaller one (yellow color) to get it to catch securely.