Correct Answer: Algae

Solution : Given – Haematology : Blood :: Phycology : ?

The relationship between the word of the second pair and the given options should be the same as the first pair of words.

Option first: Diseases

The study of blood is called haematology.

Option second: Fungi

The study of fungi is called mycology .

Option third: Fishes

The study of the fishes is called ichthyology .

Option fourth: Algae

The study of algae is called phycology . Here, the relationship between the words is similar to the first pair.

Hence, fourth option is correct.