Solute is what gets dissovled, in this case the salt, and the solVENT is whatever a material is dissolved into, in this case the water.
Molarity is expressed as the number of moles of solute per volume of the solution. For example, we are given a solution of 2M NaOH this describes a solution that has 2 moles of NaOH per 1 L volume of the solution. Acids and bases can be measured through the concentrations of H+ and OH- ions in units of molarity. Hope this helps.
Arrhenius is a substance that dissociates in water to produce hydrogen ions (H+) and there is a clear answer which coincides with this definition. The correct answer is definitely b. a substance that increases the concentration of hydronium ions when dissolved in water.