Answer:
"when"
Step-by-step explanation:
The first sentence reads: "Alain and Sylvie met at art school". This indicates that when Alain was studying art at school, he met Sylvie - he didn't meet her anywhere or anytime other than then.
In order to recreate the meaning, we have the first half of the second sentence as "Alain was studying art _______ ". We need a word or two that will make this sentence read something like: "Alain was studying art at the moment when he met Sylvie". But obviously, "at the moment when" is too long, so we can simply shorten it to "when": "Alain was studying art when he met Sylvie".
Hope this helps!