if you have kickstart <= 1.3 => workbench 1.3
if you have kickstart <= 2.04 => workbench 2.1.
PS : I don't think any stock A500 shipped with kickstart 3.1. If you have 3.1 ROM you can use workbench 3.1 of course. Actually you can run wb 3.1 with 2.04 kickstart too, but you have to install it somewhere else in a kick 3.1 environment, then connect the HDD to the 2.04 machine.