What is an indicator? Give one example.

You are provided with aqueous solutions of three salts $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$. $2‐3$ drops of blue litmus solution, red litmus solution and phenolphthalein were added to each of these solutions in separate experiments. The change in colour of different indicators were recorded in the following table:

On the basis of above observations, identify $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$ from the following options:

Which of the following is not an olfactory indicator?