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

The correct answer is Option (2) → Stereo : Stereos

Plural of nouns ending in a vowel + o:

When -o comes after another vowel, the noun takes only -s. Stereo ends in -eo, so stereo → stereos. A stereo is a music system with two speakers.

Why the Other Options are Incorrect?

Option 1: Crisis : Crisises — Incorrect. Crisis ends in -is, so -is changes to -es. Crisis → Crises.

Option 3: Reef : Reef — Incorrect. Reef keeps its -f and adds -s. Reef → Reefs. A reef is a line of rocks or coral near the surface of the sea.

Option 4: Tooth : Tooths — Incorrect. Tooth has an irregular plural. The vowel inside changes. Tooth → Teeth.