Concept:A weak acid is defined by its incomplete ionization in water.
Explanation:An acid is classified by how much it dissociates into ions when dissolved in water.
A strong acid ionizes completely, releasing all its hydrogen ions.
A weak acid, however, ionizes only slightly, meaning only a small fraction of its molecules release hydrogen ions.
The rest of the acid molecules remain undissociated in the solution.
Therefore, the property that defines a weak acid is slight ionization in water.
Answer:Option D: slightly ionizes in water.