I can't say for certain whether this is a good test question. It
might depend, I think, on the particular variety of English your
teacher is aiming at.
For me, as an American, the sentence opener "It was not long before
..." is as standard as it gets. I find "It was not long until
..." strange -- not because the meaning is strange or impossible with
"until", but because it conflicts with the standard way of following
"It was not long ...". I don't know whether this turn of phrase
is considered typical in British English.
In the U.S. it would be an excellent test question. The closer
one is to mastering American English, the more likely he is to choose B.
On the basis of grammar alone, one can even argue for E. But the
point is that tests of this kind are not testing grammar alone; they
are also testing your familiarity with certain idiomatic word groups, a
familiarity you should be acquiring through reading.
CJ