How is the underlined gerund used in the sentence?

Smiling is an important step in a baby's development.

A.

direct object

B.

object of a preposition

C.

predicate nominative

D.

subject

(smiling should be underlined.)

B. It's defined as "preposition is a word that comes before one noun to show its relationship to another word in the phrase or clause." Smiling is a verb which comes first before anything else, the noun/adjective