The answer is a.
Adding impurity increases conductivity and reduces resistivity 

Doping of a semiconductor with small traces of impurity atoms generally changes the resistivity as follows:

Explanation:
Doping is the process of intentionally adding impurity atoms to a semiconductor material to modify its electrical properties. The impurity atoms used for doping are typically from Group III or Group V elements in the periodic table.

When a semiconductor is doped, the impurity atoms replace some of the original atoms in the crystal lattice structure of the semiconductor. The impurity atoms have different valence electron configurations than the original atoms, which creates either extra electrons (n-type doping) or holes (p-type doping) in the semiconductor material.

1. n-type doping:
When a semiconductor is doped with impurity atoms from Group V elements (e.g., phosphorus), which have five valence electrons, extra electrons are introduced into the material. These extra electrons increase the conductivity of the semiconductor and decrease its resistivity. Therefore, the resistivity of the semiconductor decreases when it is n-type doped.

2. p-type doping:
When a semiconductor is doped with impurity atoms from Group III elements (e.g., boron), which have three valence electrons, holes are introduced into the material. These holes can accept electrons and increase the conductivity of the semiconductor. However, the presence of holes also increases the scattering of charge carriers, which results in an increase in resistivity. Therefore, the resistivity of the semiconductor increases when it is p-type doped.

3. Intrinsic semiconductor:
In an intrinsic semiconductor, which is not doped with any impurity atoms, the resistivity is at its highest level. This is because there are no extra charge carriers to conduct electricity efficiently.

4. May increase or decrease depending on the dopant:
In some cases, the resistivity of a semiconductor may increase or decrease depending on the specific dopant used. This can occur when the dopant atoms have unique properties that interact differently with the semiconductor material. However, in general, the resistivity of a doped semiconductor follows the trends mentioned above.

In conclusion, doping a semiconductor with small traces of impurity atoms generally changes the resistivity. n-type doping decreases the resistivity, p-type doping increases the resistivity, and intrinsic semiconductor has the highest resistivity. However, there can be exceptions depending on the specific dopant used.

Doping is used to increase conductivity and hence resistivity should be decreases