* Weak Electrolytes: These substances only partially ionize (break apart into ions) when dissolved in water. This means they conduct electricity weakly.

* Strong Electrolytes: These substances fully ionize in water, leading to strong conductivity.

Let's analyze the options:

* A. Sodium hydroxide (NaOH): A strong base, fully ionizes in water.

* B. Potassium (K): A metal, readily forms ions in water.

* C. Calcium (Ca): A metal, readily forms ions in water.

* D. Ammonia (NH₃): A weak base. While it reacts with water to form some hydroxide ions (OH⁻), it doesn't ionize completely.

In summary: Ammonia is the only substance listed that is a weak electrolyte due to its partial ionization in water.