Hydronium ions are formed by the reaction between :

According to Section 2.2.1 of the NCERT text, hydrogen ions ($H^+$) cannot exist alone; they exist only after combining with water molecules to form hydronium ions ($H_3O^+$). This reaction occurs when an acid like hydrogen chloride ($HCl$) dissolves in water:

$$HCl + H_2O \rightarrow H_3O^+ + Cl^-$$

• Option (c) is correct because the reaction between hydrogen chloride gas and water directly produces hydronium ions ($H_3O^+$) and chloride ions ($Cl^-$).
• Option (a) is incorrect because sodium hydroxide ($NaOH$) is a base and dissociates in water to produce hydroxide ions ($OH^-$), not hydronium ions:

$$NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$$

• Option (b) is incorrect because calcium chloride ($CaCl_2$) is a salt and does not generate hydronium ions in water.
• Option (d) is incorrect because ethanol (alcohol) does not dissociate to produce $H^+$ or $H_3O^+$ ions in water, as shown in Activity 2.8 where alcohol solutions do not conduct electricity.