It says right on the rubber reservoir cap to use DOT 3 brake fluid. So either that or the also compatible DOT 4 brake fluid.
DOT 4 brake fluid has a higher boiling point, but in the clutch system that's not really a critical concern.
Don't get either on any painted surfaces, because it makes a great paint remover. If you do get some on the paint, wash it off immediately with lots of water. But don't get water in the reservoir.