A subordinating conjunction is a conjunction that introduces a subordinate clause. For example although, because, etc. So the correct answer is because. Hope this helps! :)
Explanation: a subordinatig conjunction is a word that introduces a subordinate or dependent clause (a phrase that doesn't have complete meaning on its own), it joins the dependent clause to an independent clause (a phrase with complete meaning). In the given sentence the independent clause is "Neither recordings nor photographs can be copied or borrowed" and the subordinate clause is "because they are so fragile" they are joined by the subordinating conjunction "because."
