That's because the first and last carbon atoms cannot be branched to form an isomer . Therefore only the three middle carbon atoms can form isomers.
The quantity which is equivalent to the product of the absolute index of refraction of water and the speed of light in water system is the speed of light in vacuum.
<h3>What is the speed of light?</h3>
Speed of light is the rate of speed though the light travels. To find the speed of light in any medium, the following formula is used.

Here, (n) is the index of reaction and (c) is the speed of light in the vacuum. The speed of light in the vacuum is almost equal to the 3.0×10⁸ m/s.
Now the quantity which is equivalent to the product of the absolute index of refraction of water and the speed of light in water has to be find out.
The above formula can be written as,

Here, the product of index of refraction and speed of light is equal to the speed of light in vacuum. This will be true for water as well.
Thus, the quantity which is equivalent to the product of the absolute index of refraction of water and the speed of light in water system is the speed of light in vacuum.
Learn more about the speed of light here;
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For a reaction to occur, there should be mobility of ions in reactant side.
If the reactant is larger, its mobility will be lesser than that of smaller ones.
So reactants smaller in size have higher mobility which makes reaction faster.
Hence D is the correct option.
Hope this helps, have a great day/night ahead!