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.