Why does my electric oven present the option of 40A and 50A breakers?
There are a few different issues here:


120 vs. 240


A typical oven for installation in a system like the US (split 120/240) will use 240V for the heating elements (i.e., needs 2 hot wires) and 120V for the controls (i.e., needs 1 hot, 1 neutral). So you can't run it on only 120 or only 240. That is different from, for example, a computer power supply that might list 120 - 240 (or more typically something like 100 - 250) which means that any voltage in that range is OK. That is because a computer uses a power supply that can take "anything" and convert to a single lower voltage. An oven will actually use the power "as is" (at least for heating elements and lights) and has to get what it needs or it won't work properly.


40A/50A


An oven is designed with specific power requirements. It will not, unless damaged, exceed those requirements. In addition, as a device with a continuous load (i.e., it can be on for a while), it will actually be designed to use less (I think the normal number is &lt;= 80% of the rated current) power. So there is plenty of safety margin even at 40A, by design. As far as 50A, there are some ovens that require 50A instead of 40A. By designing a 40A oven so that it can operate safely on a 50A circuit - i.e., that it is engineered such that the breaker not tripping until 50A instead of 40A is "OK", that gives a little bit of flexibility on the installation. Otherwise, if you had a 50A circuit (both appropriate wire and a 50A breaker), you would need to replace the 50A breaker with a 40A breaker in order to install this oven - but that is not the case here and you are fine with 40A or 50A.


ground through the neutral


You didn't ask about this part, but I will tell you anyway. For historical reasons, many ovens used to have no ground and then later ground was piggybacked on the neutral wire. That is not safe (though marginally better than no ground at all) and is not recommended, even if local code permits it. If you have a hardwired oven then you need to connect the oven ground to either a ground wire or (if you have a metal junction box attached to metal conduit going all the way back to the breaker panel) to a metal box. If you have a plug-in installation then you need to use a properly wired 4-wire socket (NEMA 14-50) connected to 2 hots, 1 neutral and 1 ground (ground again can be metal box/conduit). If you have an older 3-wire socket, now is the time to replace it - and yes that may require running a new cable.