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

The correct answer is Option (3) → Sheaf : Sheaves

Plural of nouns ending in -f:

Many nouns ending in -f change -f to -ves in the plural, as in leaf → leaves. So sheaf → sheaves. A sheaf is a bundle of grain stalks or of papers tied together.

Why the Other Options are Incorrect?

Option 1: Diagnosis : Diagnosises — Incorrect. Nouns ending in -is change -is to -es. Diagnosis → Diagnoses.

Option 2: Monarch : Monarches — Incorrect. Here -ch sounds like “k”, so the word takes only -s. Monarch → Monarchs.

Option 4: Tooth : Tooths — Incorrect. Tooth has an irregular plural. Tooth → Teeth.