In "Dulce et Decorum Est," the "lie" is old because it has been told: -by old men -by old women -for a long time -long ago -none of these
This is actually a bit more complicated than it at first appears to be. It is of course in the final stanza that the speaker of the poem, having moved through the different persons, starting off with third person, then moving into first person, then lastly moving into second person to angrily accuse the audience of the poem of spreading the "lie" of the title with such "high zest":
My friend, you would not tell with such high zest
To children ardent for some desperate glory,
The old Lie...
Thus it appears that out of the possible answers you give the best one is that the lie has been told for a long time, and so because of this the lie is "old." Thus it is that youths such as those depicted in the first three stanzas are still conned into going off to fight for their country based on old, innaccurate notions such as glory and honour, when in fact the reality is completely different, as indicated by the gassed soldier.