In Clement Clarke Moore's poem*, "A Visit From St. Nicholas," the sentence which you refer to is:
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 1823.
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.)