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

Hair:Hair

The correct answer is OPTION 4 - Hair:Hair

• Stomach → Stomachs (not Stomaches)

• Monarch → Monarchs (not Monarches)

• Reef → Reefs (not Reef, as "reefs" is the plural form)

• Hair → Hair (correct, as "hair" is usually uncountable and remains the same in singular and plural forms)