Coloured petals of some flowers indicate the presence of acid or base in a solution. An example of such flowers is [[1]]

Concept:  Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta, and is commonly used as an indicator.
There are many other natural materials like red cabbage leaves, turmeric, and coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a solution. 
Hence the correct answer is Petunia.