Indicators indicate the presence of acid or base by showing the change in:
The correct option is C colour Indicators are compounds that help us identify whether a given substance is acidic, basic or neutral by themselves undergoing a colour change. These indicators are themselves either weakly acidic or basic in nature.
These indicators undergo colour change in solutions having different pH.
For example, Universal indicator changes it's colour to red in acidic medium, purple in basic medium and green in neutral medium.