What must a substance be able to do in order to be used as an acid base indicator

The substance must be able to alter its physical characteristic (for example, its color) in accordance to a change in pH. One example of this is litmus paper, which becomes red under acidic conditions and blue under basic conditions.

i wish i could with all of them i can barley see the first one so here's the answer to the first one b

answer; projective;