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 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.

The correct answer is gerund as the subject