The electrical conductively of pure germanium can be increased by
For A
Increasing the temperature increases the breaking of the covalent bonds and generation of more electron-hole pairs. Thus, increasing the conductivity.
For B and C
Since doping will increase the no. of charge carriers, therefore the conductivity will increase.
For D
Irradiating UV light on germanium will provide energy enough to break the covalent bond and thus create an electron-hole pair. Thus increasing the electrical conductivity.