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

Diagnosis:Diagnoses

The correct pair of singular and plural is:

☀ Diagnosis:Diagnoses

Explanation for each option:

☀ Monarch:Monarches: This is incorrect. The singular form of "Monarch" is "Monarch," and the plural form is "Monarchs."

☀ Diagnosis:Diagnoses: This is correct. The singular form of "Diagnosis" is "Diagnosis," and the plural form is "Diagnoses."

☀ Echo:Echos: This is incorrect. The singular form of "Echo" is "Echo," and the plural form is "Echoes."

☀ Hero:Heros: This is incorrect. The singular form of "Hero" is "Hero," and the plural form is "Heroes."