In which case is [H3O+] less than [OH-]?

In a basic solution, the concentration of hydroxide ions (OH-) is greater than the concentration of hydronium ions (H3O+). This relationship can be understood through the concept of the ion product of water, which at 25°C is equal to (1.0 \times 10^{-14}). This means that the product of the concentrations of H3O+ and OH- in pure water is constant.

In a basic solution, the pH is greater than 7. As pH increases, the concentration of H3O+ decreases while the concentration of OH- increases. Thus, in such environments where the solution is basic, you will always find that [H3O+] is less than [OH-]. This is a characteristic of solutions where the concentration of hydroxide ions dominates, confirming that the correct answer is indeed related to the properties of basic solutions.