The answer is C.) Stella started to beat around the bush.
The first thing we need to realize is what an idiom is. An idiom is a group of words that symbolize a meaning not otherwise implied by the actual words that are expressed. That might not have made sense, so here are some examples of idioms:
raining cats and dogs (raining hard, not actual cats and dogs)
bought the farm (refers to someone dying, not buying a literal farm)
That being said, when we say someone is beating around the bush, they are avoiding having to do or say something, which is what we're looking for. So, the third option, C, is our answer
Thank you for your question! I hope this helped! Have an amazing rest of your weekend and feel free to let me know if you need any further help! :D