Answer:
I believe the dependent marker is "When"
Step-by-step explanation:
because "He told his friends about his plan for the dead mouse." is an independent clause, and
"A dependent marker word is a word added to the beginning of an independent clause that makes it into a dependent clause."
When adding "when" to "He told his friends about his plan for the dead mouse", it makes it "When he told his friends about his plan for the dead mouse". This is an incomplete sentence and therefore now a dependent clause. As you can see, the word "when" made the first part of the sentence a dependent clause instead of independent, which it would be without "when".