The correct answer is that the rose-briar and the holly-tree are conventional symbols in this poem by Emily Brontë.
They are conventional symbols because they follow established conventions that can be recognized by the readers. The rose-brior represents love in this poem, so Brontë uses the image of a rose, a recognized symbol of love, and she describes it as wild and blooming to represent the blooming of love and passion and how with time, love - and the rose - will whither.
On the contrary, friendship is like the holly-tree, firmer and with stronger roots that take longer to grow but will not whither easily.