Worst of all: could be "der Liter", "das Liter" - both singular and "die Liter" - plural. Did not test those either.
I just tested 'das Liter' (for science!) and it's marked wrong. I'll report it.
I thought it was das Liter because I just learned the word Liter a second ago and I SWEAR it was neuter. Am I mistaken? What am I missing? If it is neuter why can it be "der"?
And I swear it's both :) See http://www.duden.de/rechtschreibung/Liter
"der Liter", maskuline or "das Liter", neuter. It is just both. But if you learn neuter and stick to it, you won't be wrong. So there is nothing to fear. I think the discussion is mainly due to Duolingo flagging something wrong during a test even though it should be right too.
Wikipedia tells us that DIN 1301, Teil 1 uses "das Liter", so for science and technology the neuter version is ok.