Final answer:
Increasing temperature will increase the percent of NH₃ converted to the ammonium ion in water.
Step-by-step explanation:
The percent of NH₃ that is converted to the ammonium ion in water can be increased by increasing temperature.
When the temperature is increased, it provides more energy to the system, allowing the reaction to occur more quickly and reach equilibrium faster.
This means that more NH₃ molecules will convert to the ammonium ion in water.
So, the correct answer is c) Increasing temperature.