Correct answer is (d) Mixing coils and delay coils
The best explanation: Two types of coils are employed in the system—mixing coils and delay coils. Coils are glass spirals of critical dimensions, in which the mixing liquids are inverted several times, so that complete mixing can result. Mixing coils are used to mix the sample and/or reagents. Delay coils are employed when a specimen must be delayed for the completion of a chemical reaction before reaching the colorimeter.