Which does the following sentence contain?

Walking by the river is our favorite pastime.

participle

infinitive as subject

gerund as subject

prepositional phrase as adjective

The correct answer is gerund as the subject

The sentence contains a gerund as a subject. The subject in this sentence is "Walking by the river" and this phrase contains the gerund "walking". Gerunds are words that take the verb form ending in - ing, but functions as a noun in the sentence.