Famed astronomer Galileo began using a telescope to explore the mysteries of space and make discoveries that religious leaders of his time frowned upon.

Identify the essential appositive.

Galileo.

Explanation:

An appositive is exemplified as the noun or the noun phrases that function to rename or redefine the noun it follows. The primary purpose of employing appositives is either to add a description (either additional or necessary) or explanation about the noun preceded by it that clarifies or explains the meaning of the phrase.

In the given sentences, 'Galileo' is the essential appositive which functions to rename the noun 'famed astronomer' it follows. The primary function of employing such an appositive is to add essential details and descriptions about the noun ('famous astronomer') that aids to clarify its meaning. Therefore, 'Galileo' is the correct answer.