a) Conventional.
A conventional symbol is a setting, object, character, or event in a piece of writing that carries a literal meaning and a figurative meaning standardized, or agreed upon in one or more societies, that is to say, it's the same for a group of people. For example, the color red is a conventional symbol, since it is generally accepted that it symbolizes blood, passion, emotion, danger, or daring.
Both the wild rose-briar and the holly-tree are conventional symbols because it is generally accepted that a rose is a symbol of love and beauty. The word "wild" stresses the intensity and passion of that "rose". As for the holly-tree, it represents peace, goodwill, and, due to its resistance to lightning, protector or guardian of evil spirits or bad luck, for example.