Here's why:
* Indirect objects are the recipients of the direct object. They answer the question "to whom?" or "for whom?"
In the sentence "They gave bonuses,"
* They is the subject.
* Bonuses is the direct object (what was given).
* The implied indirect object is "them" (the people who received the bonuses).
Let's analyze the other sentences:
* a) It is difficult to save money: This sentence has no indirect object.
* b) You worked hard each day: This sentence has no indirect object.
* d) Spent immediately: This sentence is incomplete and doesn't have a subject or an indirect object.