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

The correct pair of singular and plural is:

☀ Reef:Reefs

Explanation for each option:

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

☀ Axis:Axises: This is incorrect. The correct plural form of "Axis" is "Axes."

☀ Stomach:Stomaches: This is incorrect. The correct plural form of "Stomach" is "Stomachs."

☀ Reef:Reefs: This is correct. The singular form of "Reef" is "Reef," and the plural form is "Reefs."