Bathroom fan/light replacement - wiring - two switch to one switch options

Replacing bathroom fan/light that was wired for two wall switches. So four wires coming from ceiling. New light/fan is a one switch option, which is fine, but what are the options here. Just wire up one (red or black) and dead end the other with a wire nut? What happens if I connected both red and black to the new fixture? Would either switch then work or does the house burn down?