Ed M. answered 02/18/16
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Help with grammar, French, SAT Writing, the TOEFL and ESL.
The only gerund phrase is chewing on the slipper found in a) (here actually chewing on is arguably a two-word transitive verb whose direct object, the slipper, is thus obligatory).
In b) and d), barking and driving respectively are not gerunds but the -ing forms of main verbs in the progressive aspect (as evidenced by the presence of the respective was and is preceding auxiliaries), whereas while Loitering in c) is in fact a gerund it is, in the words of your question, "a gerund by itself" (though I would maintain it is still a gerund phrase, just one that happens to consist of a single word).