Participles are verb forms that can be used as adjectives, to create verb tense, or to create the passive voice. There are two types of participles:
present participle - they end with -<em>ing;</em>
past participle - they end in <em>-ed, -d, -t, -en, or -n.</em>
These endings make it easier for us to recognize these words. In the given sentence, we have the participle <em>fighting</em>. It's a part of the verb phrase<em> is fighting. </em>
A gerund is a noun derived from the verb by adding <em>-ing</em> to its root. Since we can see that <em>fighting </em>isn't a noun here, we know that gerund is the wrong option.
