If you spill a chemical toilet cleaning liquid on your hand, your first aid would be:

First, we recall the basic neutralisation principle of chemistry: an acid reacts with a base to give a salt and water. Mathematically, this is written as $$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}.$$

Toilet cleaning liquids that are marketed for removing stains and scales almost always contain strong mineral acids, most commonly hydrochloric acid $$\text{HCl}$$. Hence, if such a liquid is spilt on the skin, the substance on the hand is essentially an acid.

Our objective in first aid is to neutralise this acid in the safest possible way, without introducing another chemical that can itself damage the skin. Let us examine every option in turn.

Option A: Vinegar. Vinegar is mostly acetic acid $$\text{CH}_3\text{COOH}$$, which is also acidic. Adding one acid to another does not neutralise; instead, it simply increases the total acidity. So vinegar will not help in this situation and may even worsen it.

Option B: Aqueous NaOH. Sodium hydroxide $$\text{NaOH}$$ is indeed a base, and it would neutralise $$\text{HCl}$$ according to the equation $$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}.$$ However, $$\text{NaOH}$$ is a very strong base and is itself highly corrosive. Splashing a strong alkali on the skin can cause severe alkaline burns, so this is not a safe first-aid choice.

Option C: Aqueous NaHCO$$_3$$. Sodium hydrogen carbonate, commonly called baking soda, is a weak base. When it meets hydrochloric acid, the reaction is

$$\text{HCl} + \text{NaHCO}_3 \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \uparrow.$$

This neutralisation is gentle, because $$\text{NaHCO}_3$$ is only mildly basic. The products—common salt, water, and carbon dioxide—are harmless to the skin. Therefore, washing the affected area with a dilute solution of $$\text{NaHCO}_3$$ is the recommended and safest first-aid treatment.

Option D: Aqueous NH$$_3$$. Ammonia solution is a stronger base than $$\text{NaHCO}_3$$ and releases irritating fumes. Although it could neutralise the acid, it can also irritate or burn the skin and eyes. Hence, it is not preferred.

Among the four options, only the mild base $$\text{NaHCO}_3$$ offers effective yet gentle neutralisation without introducing additional hazards.

Hence, the correct answer is Option C.