Job Roles

A Perch is a fresh water fish. Similarly, a cod is a salt water fish.

Hence, Perch : Fresh Water :: Cod : Salt water.

A machine is made to work by a man. Similarly, a slave is made to work by a master.

Hence, Man:Machine::Master:Slave.

Porcupine is a rodent and mildew is a fungus.

Hence, Porcupine : Rodent :: Mildew : Fungus.

Second is a bony protective covering for the first. Here pericardium protects Heart and Cranium protect Brain.

Hence, Heart : Pericardium :: Brain : Cranium.

The words in each pair are opposites of each other. 

Hence, Dilatory : Expeditious :: Direct : Circumlocutory.

The words in each pair are opposites of each other.

Here Mature and Regressed are opposite to each other. Then the required word should be opposite of varied and it is monotonous.

Hence, Mature : Regressed :: Varied : Monotonous.

First is used to makes the second.

Hence, Stee : Rails :: Alnico : Magnets.

Chromite is a mineral of chromium and ilmenite is a mineral of titanium