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

Sheaf:Sheaves

The correct pair of singular and plural is:

☀ Sheaf:Sheaves

Explanation for each option:

☀ Diagnosis:Diagnosises: This is incorrect. The correct plural form of "Diagnosis" is "Diagnoses."

☀ Monarch:Monarches: This is incorrect. The correct plural form of "Monarch" is "Monarchs."

☀ Sheaf:Sheaves: This is correct. The singular form of "Sheaf" is "Sheaf," and the plural form is "Sheaves."

☀ Tooth:Tooths: This is incorrect. The correct plural form of "Tooth" is "Teeth."