On the common app, for the “elaborate on an activity” question, can I copy and paste the same reply (my own) for every college, or will I get marked for plagiarism? Please help!
Thank you 
Yes. It is your original work so no problem.
You can’t plagiarize your own original content
Plagiarism - the practice of taking someone else’s work or ideas and passing them off as one’s own.
I haven’t looked at the Common App in a while, but aren’t Activities something you only fill out once and it’s sent to every school? Each school has their own supplemental questions, but multiple lists of activities and descriptions defeats the purpose of a “Common” application, no?