Final answer:
In the sentence, the phrase 'her husband' is the b. indirect object, as it represents the recipient of the direct object 'a new leather wallet'.
Step-by-step explanation:
In the sentence provided, "On a whim, Nellie bought her husband a new leather wallet," the phrase in bold, 'her husband', is an indirect object. According to traditional grammar, the indirect object is the recipient of the direct object. In this case, 'her husband' is the recipient of the 'new leather wallet', which is the direct object. The sentence follows the second transitive-verb pattern (subject) + (transitive verb) + (direct object [NP]) + (indirect object [NP]). Therefore, 'her husband' is not the direct object but the indirect object because it receives the direct object.