Which of the following is a gerund phrase?

A gerund phrase is a group of words consisting of a gerund (a verb ending in -ing that functions as a noun) and its modifiers or objects. The entire phrase acts as a noun in a sentence.

• The correct answer, Running a marathon, is a gerund phrase. "Running" is the gerund, and "a marathon" is the direct object of the gerund. The whole phrase functions as a noun, for example, "Running a marathon is a challenging goal."
• "Running swiftly" contains an adverb ("swiftly") modifying the gerund, but lacks an object, making it less of a complete noun phrase.
• "To run swiftly" is an infinitive phrase, identified by "to" followed by the base verb.
• "To have run a marathon" is a perfect infinitive phrase, not a gerund phrase.

Identifying gerund phrases is crucial for understanding sentence structure.