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Question: As the acidity of a solution increases 1\. Hydrogen ion concentration increases 2\. Hydroxyl ion...

As the acidity of a solution increases
1. Hydrogen ion concentration increases
2. Hydroxyl ion concentration decreases
3. Both 1 and 2
4. None of the above.

Explanation

Solution

Dissociation of acid takes place in the following method and there is a release of Hydrogen ion. This increases acidity of solution.
H+=Hydrogen   ion{H^ + } = Hydrogen\,\,\,ion
Dissociation of base takes place in the following method and there is a release of Hydroxyl ion. This increases basicity of solution.
OH−=Hydroxyl  ionO{H^ - } = Hydroxyl\,\,ion

Complete step by step solution:
Let us first understand the terms used in the question.
Acidity is the measure of the strength of an acid or we can say, by measuring the acidity, we come to know how strong the acid is.
General dissociation reaction of an acid:
H+=Hydrogen   ion{H^ + } = Hydrogen\,\,\,ion
Let us notice how acids disassociate with an example
HCl→H++Cl−HCl\xrightarrow{{}}{H^ + } + C{l^ - }
Where: HCl=Hydrochloric acid,H+=proton/Hydrogen ion,Cl−=Chloride ionHCl = Hydrochloric\,acid,{H^ + } = proton/Hydrogen\,ion,C{l^ - } = Chloride\,ion
Every acid when disassociate, it gives H+=Hydrogen   ion{H^ + } = Hydrogen\,\,\,ion
So we can say, the strength of an acid can be measured by the concentration of H+{H^ + } ions in the solution, More the number of H+{H^ + } , more is the acidity.
Basicity is the measure of the strength of a base, by this value we come to know how strong the base is.
General dissociation reaction of a base:

OH−=Hydroxyl  ionO{H^ - } = Hydroxyl\,\,ion
Let us see how base disassociate with an example:
NaOH→Na++OH−NaOH\xrightarrow{{}}N{a^ + } + O{H^ - }
NaOH=Sodium Hydroxide,Na+=Sodium ion,OH−=Hydroxyl ionNaOH = Sodium\,Hydroxide,N{a^ + } = Sodium\,ion,O{H^ - } = Hydroxyl\,ion
Every acid when disassociated, it gives OH−=Hydroxyl  ionO{H^ - } = Hydroxyl\,\,ion .
Hence we can say that the strength of a base or basicity increases when there are more numbers of OH−O{H^ - } ions.
We can reverse this to say that, The acidity decreases when the number of OH−O{H^ - } ions increase.
Hence, Option 3 is correct.

Note:
-The concept of the number of H+{H^ + } and OH−O{H^ - } ions being used to determine the strength of a solution is only valid for Arrhenius and Bronsted Acid and base.
-Acidity of compounds like AlCl3AlC{l_3} cannot be determined on the basis of H+{H^ + }concentration. These are Lewis acid and bases.