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.