Final answer:
An Arrhenius base is defined as a substance that ionizes to yield hydroxide ions (OH−) in aqueous solution. The correct answer to the question is option 2) 'It produces OH⁻ in water'.
Step-by-step explanation:
The definition that best describes an Arrhenius base is: It produces OH− in water. According to Svante Arrhenius, an Arrhenius base is a substance that dissociates in water to produce hydroxide ions (OH−).
This is a specific definition that differentiates it from the broader Brønsted-Lowry definition, which defines a base as a proton acceptor, and from the Lewis definition, which considers a base as an electron-pair donor.
While an Arrhenius acid is known for producing hydrogen ions (H+), the distinguishing feature of an Arrhenius base is its ability to produce hydroxide ions, making option 2) 'It produces OH− in water' the correct choice. This definition strictly applies to the behavior of substances when dissolved in water, where the base increases the concentration of hydroxide ions.