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

The correct answer is Option (2) → Torpedo:Torpedoes

The correct pair of singular and plural is Torpedo:Torpedoes

Explanation for each option:

 Torpedo:Torpedoes: This is correct. The singular form of "Torpedo" is "Torpedo," and the plural form is "Torpedoes."

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

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

 Reef:Reef: This is incorrect. The plural form of "Reef" is "Reefs."