your hand if you didn't know ALT on the transponder needed to be selected for ATC to get your mode C.

It's fine, we have all done it....flown around with the supid thing set to the ON position and ATC starts bitching that they are not getting altitude....and then the Copilot sneak-aly switches the radio to ALT from ON and now it's working just fine for the rest of the day....

I at one point was that person, but by my 2nd or 3rd flight in an R22 the CFI explained that to me... granted that aircraft didnt have an avionics master and it was the older style Bendix King KT 76A that had roller knobs for the 4 digits plus the mode selector.  You had to turn it from off, through standby, and on, to get to ALT. That process immediately ended up with the CFI having to explain what those positions meant.  If the aircraft you learned to fly in and everything after had a different setup maybe you missed that moment but I would have thought somewhere in ground school, going through transponder requirements and therefore transponder types and modes, you would have covered it.