The correct answer is A. Subordinating conjunction
Step-by-step explanation:
In grammar, the term subordinating conjunction refers to a word mainly used to link a dependent clause, which is a sentence that is not complete and needs from another sentence to stand, to an independent or complete clause. This includes words such as before, if, although, because or when that establish a subordinating or dependence relationship. In the case of the sentence "Please put the toys away when you are finished with them", there is an independent clause "Please put the toys away" and one dependent clause "when you are finished with them", this means the word "when" which is the word in bold mainly works as a subordinating conjunction as it links a dependent clause to an independent one and establishes the relationship between both. Therefore, the word in bold "when" is being used as a subordinating conjunction.