Which of the following is a correct pair of singular and plural?

The correct answer is Option (2) → Diagnosis:Diagnoses

Plural of nouns ending in -is:

Nouns ending in -is change -is to -es in the plural. So diagnosis → diagnoses. A diagnosis is a doctor’s finding of what illness a person has.

Why the Other Options are Incorrect?

Option 1: Monarch:Monarches — Incorrect. Here “ch” sounds like “k”, so the noun takes only -s. Monarch → Monarchs.

Option 3: Echo:Echos — Incorrect. Echo is one of the nouns ending in -o that take -es. Echo → Echoes.

Option 4: Hero:Heros — Incorrect. Hero also takes -es. Hero → Heroes.