In Clement Clarke Moore's poem, "A Visit From St. Nicholas," "ere" (line 55) means: as, when, before, since or none of these.

In Clement Clarke Moore's poem*, "A Visit From St. Nicholas,"
the sentence which you refer to is:

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But I heard him exclaim, ere he drove out of sight
—
Happy Christmas to all, and to all a good
night.

This is a difficult question
because the word "ere" is archaic: it is not used anymore, though it
was

In answering the question,
we must look at the responses you have to choose from. The "throw-away" answer is probably
"since;" using it in place of "ere" sounds awkward, and it if St. Nick is in the
process of leaving, "since" would not make sense because it deals with the
past. E.g., "Ever since I broke my ankle, skating has been hard for me.) This is the most common
use of "since" in my experience (though some people use "since" as
"because").

For this particular question, you need to
know the definition of "ere" before you can select from the other three, for the three
answers sound as if they might make sense. There are no context clues to use, or root words to
give us a hint as to what "ere" means. However, if you look the word up in the dictionary, you
will find your answer. (Online, when I double-checked, I had to go to a second source because
none of the answers provided for your question were given at the first site.) "Ere" comes from
the Middle English, before the year 900. It is a very old word. And in
answer to your question, it means "before."

(*Note: there are those
who think Henry Livingston, Jr. might have written the poem, but it is generally credited to
Clement Clarke Moore.)