Find out relation between first two words and choose the same relationship to the third word, from the alternative.

Haemoglobin : Iron :: Chlorophyll : ?

Magnesium

Haemoglobin : Iron

Iron is the main component of Hemoglobin.

Similarly ,

Chlorophyll :

Main component of Chlorophyll is Magnesium.

So, Chlorophyll : Magnesium

The correct answer is Option (2) → Magnesium