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

The correct answer is Option (3) → Synopsis : Synopses

Plural of nouns ending in -is:

Nouns ending in -is change -is to -es in the plural. So synopsis → synopses. A synopsis is a short summary of a story or a book.

Why the Other Options are Incorrect?

Option 1: Piano : Pianoes — Incorrect. Piano takes only -s. Piano → Pianos.

Option 2: Hero : Heros — Incorrect. Hero is one of the nouns ending in -o that take -es. Hero → Heroes.

Option 4: Leaf : Leafs — Incorrect. In leaf the -f changes to -v before -es. Leaf → Leaves.