Final answer:
CN−, or the cyanide ion, will be basic in aqueous solution because it is the conjugate base of a weak acid (HCN) and can generate hydroxide ions in water.
Step-by-step explanation:
The ion that will be basic in aqueous solution is CN− (cyanide ion). This is because it is a conjugate base of a weak acid, HCN (hydrogen cyanide), which can react with water to produce hydroxide ions (OH−) and thus increase the pH of the solution, resulting in a basic solution. In contrast, ClO4− (perchlorate ion) and Cl− (chloride ion) are the conjugate bases of strong acids (HClO4 and HCl, respectively) and do not have basic behavior in solution.
The C2H5NH3+ (ethyl ammonium ion) is the conjugate acid of a weak base and would produce an acidic solution. From the given options, the ion C2H5NH3+ is the conjugate base of the weak acid C2H5NH2, so it will be basic in aqueous solution.