Which of the following is used as an acid-base indicator?

The ones that change visibly when exposed to acid or bases and the two changes are distinct from one another are indicators of acid and bases.
Since it could skew the results, an indicator of acid and bases shouldn't be acidic or basic in nature.

We must understand what acids and bases are in order to be able to respond to this question. The terms "acid" and "base" can be defined in a variety of ways.
Any species that has the ability to take electrons is referred to as a Lewis acid, and any species that has the ability to donate electrons in order to become a more electrically stable species is referred to as a Lewis base.
According to the Bronsted-Lowry definition of acids and bases, any species in a solution that may release protons and bears the chemical symbol H⁺ is considered an acid. And bases are any species that have the ability to take protons.

The release of protons and hydroxide ions is also the foundation of the Arrhenius concept of acid and base. In a solution, acids are the species that released protons, while bases are the species that contributed hydroxide ions.
We will now evaluate each of the possibilities given to us in the question one at a time to determine which one is best.

The first choice is potato; as we all know, potatoes are formed of starch, which may best be used to determine the presence of iodine but not of acid or base. The first choice is thus erroneous.

The tomato in the second choice is acidic by nature since it includes ascorbic acid, a weak acid. As a result, it cannot reveal the existence of acid or base because doing so would affect the outcomes.
When exposed to acidic or neutral solutions, turmeric turns yellow; when exposed to basic ones, it turns brilliant red. It can certainly be used as an indicator, then. Therefore, it is the best choice.
The protons of the acid would not dissociate in dry litmus, preventing it from serving as an indicator as per the last choice, and therefore it would not produce the expected results. Consequently, this choice is likewise untrue.

Hence, the correct answer is C.