State the effect of concentration of H⁺ (aq) ions on the nature of the solution. Do basic solutions also have H⁺ (aq) ions ? If yes, then why are these basic?

 The concentration of H⁺ (aq) ions determines the acidic nature of the solution. Hence, When the concentration of hydrogen H+ (aq) ions increases the solution becomes more acidic while when the hydrogen H+ (aq) ion decreases cause an increase in the basicity of the solution.

Yes, Basic solutions also have H+ (aq) ions. But still, they act as a base because the concentration of hydroxide (OH^-) ion is more as compared to hydrogen H+ (aq)  ion concentration.