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

Phenomenon:Phenomena

The correct pair of singular and plural is:

☀ Phenomenon:Phenomena

Explanation for each option:

☀ Phenomenon:Phenomena: This is correct. The singular form of "Phenomenon" is "Phenomenon," and the plural form is "Phenomena."

☀ Piano:Pianoes: This is incorrect. The singular form of "Piano" is "Piano," and the plural form is "Pianos."

☀ Echo:Echos: This is incorrect. The singular form of "Echo" is "Echo," and the plural form is "Echoes."

☀ Hero:Heros: This is incorrect. The singular form of "Hero" is "Hero," and the plural form is "Heroes."