The answer is B:
Some verbs can be transitive and intransitive.
In the sentence B, gathered is an intransitive verb because it doesn't require a complement or an object, the sense of the sentence is complete.
In A, the verb gathered is transitive and the troops is the direct object.
In C, the verb gathered is transitive and their remaining courage is the direct object.
In D, the verb gathered is transitive and food and supplies is the direct object.